Signal (South Korean TV series)

For the Japanese remake, see Signal (Japanese TV series).

Signal
Promotional poster
Hangul	시그널
Revised Romanization	Sigeuneol
Genre	Procedural Drama Thriller Fantasy
Created by	Choi Jin-hee Park Ji-young
Based on	Frequency by Toby Emmerich
Written by	Kim Eun-hee
Directed by	Kim Won-seok
Starring	Lee Je-hoon Kim Hye-soo Cho Jin-woong
Opening theme	"I Will Forget You" by Jung Cha-sik
Ending theme	Various songs
Composers	Kim Joon-seok Park Sung-il
Country of origin	South Korea
Original language	Korean
No. of seasons	1
No. of episodes	16 (list of episodes)
Production
Executive producers	Lee Chan-ho Lee Sang-baek
Producers	Lee Jae-moon Park Eun-kyung
Cinematography	Choi Sang-mook Lee Joo-young
Editor	Kim Na-young
Camera setup	Single camera
Running time	64–88 minutes
Production companies	AStory (season 1) B.A. Entertainment (season 2)
Original release
Network	tvN
Release	January 22, 2016 (2016-01-22) – present (present)
Related
Signal (2018) Unknown Number (2019)

Signal (Korean: 시그널) is a South Korean television series written by Kim Eun-hee, directed by Kim Won-seok, and starring Lee Je-hoon, Kim Hye-soo, and Cho Jin-woong. Based on the 2000 film Frequency in premise and inspired by real-life criminal incidents in Korea, including the Hwaseong serial murders. The first season aired on tvN from January 22 to March 12, 2016, every Friday and Saturday at 20:30 (KST). In March 2024, a second season was confirmed in production.

The series received widespread acclaim from audiences and critics alike for its story and performances. It is also one of the highest-rated Korean dramas in cable television history with a peak audience viewership of 12.544%.

Plot

In present-day 2015, criminal profiler Park Hae-young (Lee Je-hoon) finds a mysterious discarded walkie talkie; inexplicably, the person at the other end, Detective Jae-han (Cho Jin-woong), is from the past, and addresses him familiarly, even though Hae-young has never met him. Hae-young is able to solve a 15-year-old kidnapping and murder case involving a former classmate from his childhood with Jae-han's help. The success of this case triggers the formation of a cold case team, led by Detective Cha Soo-hyun (Kim Hye-soo), who has been searching for Jae-han, her long-lost mentor and unrequited love, for the past 15 years.

With Jae-han's help from 1989, Hae-young solves other cold cases that have remained unsolved for years and is drawn into the mystery surrounding Jae-han's disappearance in the year 2000. As they try to fix the past and catch the criminals, unintended consequences follow in the present.

Cast

Main

• Lee Je-hoon as Park Hae-young

The jaded and hotheaded criminal profiler who finds the walkie-talkie by chance, and uses his connection with Jae-han to solve cold cases.

• Kim Hye-soo as Cha Soo-hyun

The first female police officer in the Special Task Force, she becomes the leader of the Seoul cold case squad. She has been searching for Jae-han, her mentor and unrequited love, for 15 years, believing him to be dead.

• Cho Jin-woong as Lee Jae-han

The socially inept but virtuous and quick-tempered detective from the past who establishes a rapport with Park through the walkie-talkie. His timeline during his communication with Park spans from 1989 to 2000.

Supporting

Police officers

• Jang Hyun-sung as Kim Bum-joo

The corrupt police superintendent.

• Jung Hae-kyun as Ahn Chi-soo

The section chief and supervisor of the Seoul cold case squad who acts as Bum-joo's lackey but appears morally gray.

• Kim Won-hae as Kim Gye-chul

A police officer who joins the Seoul cold case squad and often serves as comic relief.

• Jung Han-bi as Oh Yoon-seo

A colonel.

• Lee Yoo-jun as Jung Han-ki

A forensic scientist and member of the Seoul cold case squad, often seen with Detective Kim.

• Kim Min-kyu as Hwang Eui-kyung

A junior officer that cleans up the Seoul cold case squad's work area and admires them.

Extended

• Kim Hyun-bin as young Park Hae-young
• Kang Chan-hee as Park Sun-woo

Hae-young's older brother who was convicted in the high school gang rape case and died by suicide after being released.

• Yang Dae-hyuk as Hong Won-seo

A detective.

• Lee Moon-soo as Lee Jae-han's father
• Seo Ju-hee as Cha Soo-hyun's mother
• Lim Hwa-young as Cha Soo-hyun's younger sister
• Lee Do-yeop as Kim Jung-jae

Special appearances

• Oh Yeon-ah as Yoon Soo-ah (ep. 1-2)
• Lee Young-eun as Kim Yoon-jung (ep. 1-2)
• Lee Si-a as Kim Won-kyung (ep. 2-4)
• Kim Jung-young as Won-kyung's aunt (ep. 2-4)
• Kim Ki-cheon as Lee Chun-goo (ep. 3-4)
• Lee Dong-ha as Han Se-kyu (ep. 5-8)
• Jung Suk-yong as Oh Kyung-tae (ep. 5-7)
• Park Si-eun as Oh Eun-ji (ep. 5-6)
• Choi Woo-ri as Shin Yeo-jin (ep. 5-6)
• Yoo Ha-bok as Shin Dong-hoon (ep. 5-7)
• Son Hyun-joo as Jang Young-chul (ep. 7, 11, 14, 16)
• Lee Eun-woo as Shin Da-hye (ep. 7-8)
• Lee Sang-yeob as Kim Jin-woo (ep. 9-11)
• Lee Chae-kyung as Jin-woo's mother (ep. 10)
• Seo Eun-ah as Yoo Seung-yeon (ep. 10-11)
• Shin Yi-joon as Kang Hye-seung (young / ep. 11-14)
• Kim Woo-suk as Lee Dong-jin (ep. 12-14)
• Lee Jin-kwon as a bully (ep. 12)
• Hwang Seung-eon as Han Do-yeon (ep. 13)
• Jeon Su-ji as Kang Hye-seung (adult / ep. 13-14)
• Seo Ji-hoon as Jang Tae-jin (ep. 14)

Episodes

No.	Title	Directed by	Written by	Original release date
1	"You're Doomed"	Kim Won-seok	Kim Eun-hee	January 22, 2016 (2016-01-22)
In 2000, 10 year old Park Hae-young is the only person to see his classmate Yoon-jung leaving school with a woman, unaware that she is being kidnapped. The suspect is male medical student Hyung-joon; though Hae-young knows the culprit was a woman, no one listens to him. Yoon-jung is murdered and Hyung-joon is never found, leaving Hae-young disillusioned with the police. 15 years later, he has become a jaded but intelligent criminal profiler. A few days before the statute of limitations on Yoon-jung's unsolved case has passed, he finds an old police walkie-talkie crackling from a trash bag of discarded evidence and intercepts a live communication from Detective Lee Jae-han in the year 2000. Jae-han has just discovered Hyung-joon's corpse and addresses Hae-young with familiarity, even though the two have never met. With Jae-han's clues, Hae-young finds the remains of Hyung-joon in the present. Supported by Detective Cha Soo-hyun, Hae-young and the team detain the culprit, only to realize minutes before the statute expires, that she has been framed by the real culprit, Nurse Yoon Soo-ah.
2	"We Still Have a Chance"	Kim Won-seok	Kim Eun-hee	January 23, 2016 (2016-01-23)
The team captures Soo-ah but Yoon-jung's statute expires. Before she can walk free, they convict her of Hyung-joon's murder. Amid public pressure from the injustice of Yoon-jung's case, the statute for cold cases is lifted, allowing criminals to be apprehended no matter how much time passes, and corrupt police superintendent Kim Bum-joo begrudgingly institutes a new cold case squad in Seoul, overseen by Chief Ahn Chi-soo and Soo-hyun. The team includes the members involved in solving Yoon-jung's case: Officer Kim Gye-chul and forensic scientist Jung Han-ki, with Hae-young as their criminal profiler. Their first case is the South Gyeonggi Nambu serial murders, the most famous cold case in Korea. Hae-young receives another walkie-talkie transmission from Jae-han in 2000, now wounded on a mountain. Jae-han warns him that he will have to convince him of something in 1989 as "they will start again", before a gunshot is heard. Hae-young is mystified when the next transmission is from the younger 1989 version of Jae-han, then a rookie helping in the Gyeonggi Nambu case. Confused, he lets slip a detail about the 8th murder that is yet to happen, allowing Jae-han to stop the murder. As a result, in 2015, all records of the Gyeonggi Nambu murders change.
3	"We Can Prevent the Killings"	Kim Won-seok	Kim Eun-hee	January 29, 2016 (2016-01-29)
In 2015, Hae-young realizes that everyone else's memories have changed to accommodate the new reality, with the 8th victim, Lee Mi-sun, now being a survivor. He alone remembers the original reality and realizes that he and Jae-han can change the past and the future together. However, he quickly learns that their actions also have consequences when the team discovers Kyung-soon, a coworker of Mi-sun, as the new 10th victim; she had not been affected by the murders in the original reality and the murderer has apparently struck again after all these years. Hae-young attempts to undo Kyung-soon's death by telling Jae-han to catch the perpetrator in the past so no one is affected in the present. In 1989, Jae-han arrests the wrong man and the killer commits the 8th murder. Due to this, Jae-han is suspended from the police force. In 1989 and 2015, Hae-young and Jae-han come to the same conclusion: the killer was forced to board the late-night bus to escape Jae-han. However, bus driver Lee Chun-goo claims that no one boarded the bus at that stop. For interfering with the investigation, Jae-han is thrown into a holding cell. Angry at Hae-young, he refuses to listen until Hae-young reveals that the 9th victim is Kim Won-kyung, a woman whom Jae-han has feelings for. Less than an hour before she is to be murdered, Jae-han breaks out to save her.
4	"We Found the Killer"	Kim Won-seok	Kim Eun-hee	January 30, 2016 (2016-01-30)
In 2015, evidence confirms Kyung-soon's murderer was bus driver Lee Chun-goo and the original South Gyeonggi Nambu killer who committed the first 9 murders was Chun-goo's son, Lee Jin-hyung, who is currently paralyzed. In 1989, Jae-han is purposely misled by Chun-goo, allowing Jin-hyung to kill Won-kyung. In 2015, Chun-goo killed Kyung-soon because she had evidence against them with which she blackmailed them for years. The evidence was the taser that Jae-han had given Won-kyung for protection, which contained Jin-hyung's DNA. These findings are presented to the press as the first solved case of the Seoul cold case squad. In custody, Chun-goo reveals that in 1989, Jae-han had realized that Jin-hyung was the real culprit; their struggle resulted in his paralysis. In a flashback from 2000, a young Hae-young discovers his older brother, Park Sun-woo, dead by suicide, having been released after being convicted in a high school gang rape case. Hae-young tells Jae-han that the case will remain unsolved in 1989 because the technology for them to prove it was Jin-hyung will not exist until the present.
5	"Changing the Past Alters the Present"	Kim Won-seok	Kim Eun-hee	February 5, 2016 (2016-02-05)
In 1995, rookie Soo-hyun becomes the first woman to join the Special Task Force. The team investigates a string of burglaries. Jae-han suspects ex-convict Oh Kyung-tae, who insists he is innocent. In 2015, Hae-young learns that Jae-han was declared missing in 2001 and discharged with allegations of bribery. Bum-joo insists that the truth about Jae-han must never be discovered. Chi-soo discovers that Hae-young found the discarded police walkie-talkie. During their next transmission, Hae-young is shocked to learn that it is now 1995 for Jae-han; even though only one week has passed for Hae-young, six years have passed for him. Jae-han is desperate to solve the burglary case but Hae-young fears changing the past could cause people who had originally lived to die. He decides giving a few clues is harmless, as the case remains unsolved in 2015. However, these clues lead Jae-han to mistakenly arrest Kyung-tae in 1995, resulting in the death of Kyung-tae's young daughter Oh Eun-ji, who had been alive in the original reality. The next day, Hae-young finds that reality has changed again: the burglary case is now considered solved in the present, with Kyung-tae imprisoned for 20 years. Kyung-tae is released from prison in 2015 and immediately kidnaps a woman named Shin Yeo-jin. Jae-han tearfully tells Hae-young that everything was his fault.
6	"Catch Him By All Means"	Kim Won-seok	Kim Eun-hee	February 6, 2016 (2016-02-06)
In 1995, Jae-han arrests Kyung-tae, causing his daughter Eun-ji to take the bus home. The bridge collapses and Eun-ji and a younger Shin Yeo-jin, are pinned. The rescue crew can only save one. Yeo-jin's father, Dong-hoon, insists they save Yeo-jin, leaving Eun-ji to die. In 2015, Kyung-tae is targeting Dong-hoon, whom he blames for his daughter's death. He leads Dong-hoon to a truck that he claims contains Yeo-jin. Hae-young realizes too late that he has rigged the truck to explode and is unable to save Soo-hyun, who dies in the detonation. In 1995 and 2015, Jae-han and Hae-young both discover that the real perpetrator is Han Se-kyu, the son of one of the burglary victims and the 1995 eyewitness, who had lied and framed Kyung-tae. In 2015, he is now a wealthy lawyer with a politically-connected father, which will make it difficult for Hae-young to capture him.
7	"We Must Dig Deeper"	Kim Won-seok	Kim Eun-hee	February 12, 2016 (2016-02-12)
In 1995, Jae-han, angry and wanting justice for Kyung-tae's false conviction, uncovers Se-kyu's car, which contains the items he stole items. Thus, Se-kyu is arrested and Kyung-tae is exonerated in 1995 rather than 2015. However, Kyung-tae still immediately goes after Dong-hoon, this time managing to murder him. He is convicted again and dies in prison, while Se-kyu himself is released after only a few months. The case is bigger than the burglaries: it involves extreme corruption within both the Korean government and the chaebols linked to the construction company that built the collapsed bridge. Jae-han's superior is replaced with Kim Bum-joo, then a senior officer working in favor of the corrupt chaebols and higher-ups. Jae-han finds that the only stolen item not found in Se-kyu's belongings was a diamond necklace stolen from Congressman Jang Young-chul. A young woman, Da-hye, stole the necklace from Se-kyu and had attempted to sell it. Jae-han tracks down Da-hye, only to find that she has died. In 2015, Hae-young finds that history has changed again; Soo-hyun is alive once more, since the 2015 events with Kyung-tae and the rigged truck never happened in this reality. Relieved, he resolves to stop changing the past, now realizing how grave the consequences can be, and disposes of the walkie-talkie. Chi-soo follows him and retrieves the walkie talkie, discovering that it's the one that belonged to Jae-han. A man visits and requests the squad to investigate his past fiancee, Da-hye, who supposedly committed suicide in 1995; however, he saw her alive recently. The squad finds evidence that confirms Da-hye never died in 1995: she is still alive.
8	"How Are You Alive?"	Kim Won-seok	Kim Eun-hee	February 13, 2016 (2016-02-13)
In the present, the squad investigates the 20 year old disappearance of Shin Da-hye. Hae-young learns that Se-kyu raped Da-hye at a party with one of his friends recording. The sex tape was kept by the other boys to blackmail Se-kyu. Se-kyu crept into each of the three boys' wealthy homes to find the tape, taking other valuables to make it look like burglaries; then framed Kyung-tae. Da-hye has been hiding in Germany under a different name for 20 years and recently returned to Korea because her ill mother needed an organ transplant. She explains that after she stole the necklace, Se-kyu broke into her apartment and murdered a woman that he believed was her; he had actually killed Da-hye's best friend, who was sleeping over. When Se-kyu covered up the murder by making it look like she committed suicide, Da-hye took on the identity of her deceased friend and went into hiding. An audio recording of the murder exists; that recording, combined with Da-hye's testimony, is used to charge Se-kyu. Se-kyu voluntarily turns himself over to interrogation while ordering Bum-joo to murder Da-hye. Soo-hyun foils the attempt. Hae-young reveals that the recording was not the whole recording but rather a copy while the original, longer version proves that Se-kyu was in the apartment that night. When Soo-hyun brings Da-hye in, a shocked Se-kyu confesses to the murder. Da-hye later explains that she had mailed the floppy disk to Jae-han's precinct in 1995. The disk was intercepted by Bum-joo, who deleted all the incriminating data about the government officers and chaebols' involvement. Though angered, Jae-han remained determined to do all he could. It is revealed that in 2000, it was Ahn Chi-soo who shot and killed Jae-han on the mountain right after Jae-han sent his last transmission. In 2015, Hae-young hears the distinctive static of the walkie-talkie and is shocked to find it in Chi-soo's desk before Chi-soo himself confronts him.
9	"Hongwon Dong Case"	Kim Won-seok	Kim Eun-hee	February 19, 2016 (2016-02-19)
In the present, Chi-soo explains to Hae-young that the walkie-talkie was Jae-han's, found with his abandoned car when he disappeared in 2001. Hae-young takes back the radio, declaring that it now belongs to him. On a visit to Soo-hyun's house, he discovers Jae-han's notebook that Soo-hyun had kept, and inside it, a handwritten note listing four cases. Hae-young notes that the first two cases are ones he and Jae-han solved together: the Gyeonggi Nambu murders in 1989 and the burglaries of 1995. The next two listed are the Hongwon-dong case of 1997 that he has never heard of and the Injoo high school gang rape case in 1999. Hae-young is shaken to discover that Jae-han had worked on the case his older brother Sun-woo was involved in. During their next conversation, it is 1997 for Jae-han and he has not heard from Hae-young for two years. Hae-young warns him of the Hongwon-dong case that is about to happen. In the present, Soo-hyun surprisingly suggests that their next investigation be the Hongwon-dong incident, revealing that two women were murdered in 1997, and a third set of remains have just been found, suggesting a serial killer. As the third deceased's time of death dates to 2001, the squad combs the mountain where the third corpse was found for more bodies. Soo-hyun reveals that in 1997, there was one additional victim: herself. In 1997, by chance Jae-han stumbles onto a case of two murders that took place in different jurisdictions; he is convinced they are the acts of the same killer. Everything about the two victims were different except that they were both targeted when walking alone at night while wearing headphones. Soo-hyun comes across Jae-han's investigative notes and takes it upon herself to retrace the victims' steps to find any overlap. While doing so she inadvertently copies the profile of the two victims, and chances upon the same convenience store that the second victim visited where the murderer, a clerk named Kim Jin-woo, works. Soo-hyun is targeted and attacked by Jin-woo and wakes up bound in a bathroom. Left unsecured, she escapes and flees out onto the streets and is found by Jae-han. In 2015, many more victims' bodies are found on the mountain.
10	"Someone Has to Stop the Bad Guys"	Kim Won-seok	Kim Eun-hee	February 20, 2016 (2016-02-20)
In 1997, the traumatized Soo-hyun considers giving up on being a cop, but on Hae-young's advice Jae-han reassures her. In 2015, a total of eight additional bodies are found on the mountain. In addition to the first corpse found that triggered the search and the two victims that were found in the streets, the Hongwon-dong killer is believed to be responsible for 11 murders. On Bum-joo's suggestion, the Hongwon-dong case is assigned to the Seoul cold case squad. Bum-joo warns Chi-soo that the penalty for failure will be severe. The respective teams in 1995 and 2015 both attempt to find Jin-woo's house via details recounted from Soo-hyun's capture; both fail. Hae-young and Jae-han share details of their respective investigations across time: Jae-han is shocked to hear that Soo-hyun becomes the leader of the cold case squad while Hae-young does not have the heart to tell Jae-han that he is missing and presumed dead in the present. On Hae-young's suggestion, the cold case squad focuses their efforts on the 11th body, the only victim they failed to identify, who was murdered in 2014 and who Hae-young believes Jin-woo had an emotional connection to. In flashbacks, it is revealed that Jin-woo was abused by his mother as a child. The resultant neuroses caused each of the various habits that signify his murders. The 11th victim was a woman, later identified by DNA as Yoo Seung-yeon, who was infatuated with Jin-woo and over the years they developed a relationship. Using forensic details of Seung-yeon's bones, Hae-young locates her former workplace and finds a journal, getting key details that allow him to deduce that the victims all shopped at the same convenience store. Hae-young visits each store in the area questioning clerks until he finds the one Jin-woo worked at. Meanwhile Soo-hyun retraces her own steps and arrives at Jin-woo's home. Finding the door unlocked, she enters.
11	"The One Last Case"	Kim Won-seok	Kim Eun-hee	February 26, 2016 (2016-02-26)
In 2015, the police ransack Jin-woo's home, finding a wealth of evidence linking him to each of the victims and even the bones of his mother kept in a closet, but Jin-woo himself has gone missing. Hae-young realizes that after Seung-yeon's murder, Jin-woo is planning to commit suicide at the same location where he buried the bodies, no longer able to murder after killing someone he had genuine affection for in Seung-yeon. Soo-hyun arrives just in time as Jin-woo hangs himself and prevents his death. The next time they speak, Jae-han learns that they have solved the case in 2015 and demands to know the identity of the perpetrator, but Hae-young refuses to answer in order to avoid changing history and causing more unexpected deaths. However, when Hae-young looks at his note of solved cases, the line for the Hongwon-dong case disappears before his eyes: history has changed again. He discovers that Jin-woo has vanished from custody and is instead in a mental institution after having been arrested by Jae-han who independently solved the case in 1998, preventing the nine additional murders that started in 2001 - each of the nine women are now alive and well. The next time the walkie-talkie is active again, it is now 1999 for Jae-han, with only one case remaining that was listed in the notebook: the Injoo high school case involving Hae-young's brother Sun-woo. Hae-young pleads with Jae-han to find the truth of the case. In 1999, then 9-year old Hae-young is living in Injoo with his mother and Sun-woo. At their high school, a female student named Kang Hye-seung attempts suicide after accusing 18 male students of gang rape. The allegation takes the form of an unusual poem that soon spreads, causing a citywide scandal. As Injoo is undermanned for such an extensive investigation, reinforcements from the special task force are requested - Jae-han interjects himself into this squad, led by Bum-joo. At Injoo, Jae-han partners with a local, a young detective named Ahn Chi-soo. After receiving a photograph from a student, Jae-han begins to suspect that the poem contained a code. In 2015, Bum-joo threatens Chi-soo, only for Chi-soo to tender his resignation. Chi-soo then calls Hae-young and warns him not to continue his investigation of the Injoo case, only for Hae-young to refuse. Chi-soo requests a meeting with Hae-young at the Injoo hospital, promising to come clean and claiming he orchestrated the entire affair, but when Hae-young arrives he finds Chi-soo seriously wounded.
12	"The Case Was Manipulated"	Kim Won-seok	Kim Eun-hee	February 27, 2016 (2016-02-27)
In 1999, Jae-han figures out the code from the poem and the photograph and shifts his focus to the Injoo high school student council, while Bum-joo works with the local chief of police to immediately evacuate all seven of those students from the city. Jae-han suspects one of the seven is the actual author of the poem with the rape allegation acting as whistleblower; this student, Lee Dong-jin, is to be spirited out of the city by Chi-soo, but Jae-han intercepts them and brings Dong-jin into custody. Dong-jin begins to recount the incident that began with Sun-woo privately tutoring Hye-seung at a remote house owned by Dong-jin's family, but the interrogation is interrupted when Dong-jin's father arrives and takes him away. The next time Jae-han is able to question any of the council students, they have already had time to prepare fabricated statements - each gives an identical testimony blaming Park Sun-woo for instigating the gang rape, even Dong-jin. Next Sun-woo himself is brought in for interrogation, and reveals that he was the one who sent the photograph to Jae-han. Investigating further, Jae-han discovers a discrepancy in the witness testimonies and after pressing one of the locals, learns that all of the witness and investigator statements were coached. In a rage Jae-han then confronts Kim Jung-jae, one of his fellow officers from Seoul and his best friend, who confesses that he was paid to cooperate with the frame, and as he had recently learned from his wife that they were about to lose their house, could not refuse. The final piece of evidence is when Hye-seung herself gives a statement naming Sun-woo. In 2015, a dying Chi-soo confesses to Hae-young that he was the one who killed Jae-han, and begs Hae-young to tell Jae-han that he had no choice. Chi-soo gives a cryptic statement claiming "it all started in Injoo", and perishes from his wound. Under interrogation, Hae-young admits that he was called there by Chi-soo to discuss the Injoo high school case which the section chief claimed was fixed. Bum-joo orders an immediate investigation of the homicide enlisting all officers, but excludes the cold case squad as Hae-young is considered the prime suspect. A witness statement providing motive for Hae-young is then given by a man named Kim Sung-bum, who was the key witness in Jae-han's bribery allegation and that Hae-young suspected of working with Chi-soo. Under pressure, Hae-young reveals to Soo-hyun that Chi-soo killed Jae-han and framed him for the bribery allegation in 2001. Later, Hae-young explains to Soo-hyun that in 1999, officers investigating the Injoo high school case arrived one day and arrested Sun-woo, later convicting him of a crime. Hae-young's father believed these allegations while their mother did not, causing a divorce. The father took the then 9-year old Hae-young with him to Seoul, the next time Hae-young saw his mother again she had committed suicide, while Sun-woo also committed suicide shortly after being released from prison. Some time later, a teenage Hae-young learns from a rumor that the key evidence leading to Sun-woo's conviction was eyewitness testimony from a student - he tracks the student down who reluctantly admits he was instructed to give the fabricated testimony by the police, this instills in Hae-young a deep-seated hatred of law enforcement. Soo-hyun tracks down Jung-jae, who had resigned from the force after returning to Seoul from the Injoo case, but even after revealing Chi-soo's death and deathbed confession of Jae-han's slaying, Jung-jae refuses to talk. When Hae-young next returns to the precinct, by chance he witnesses Sung-bum leaving and realizes he saw Sung-bum's car outside the Injoo hospital. Soo-hyun and Hae-young find and investigate a house listed under Sung-bum's mother's name that Hae-young believes may have been used to store evidence, and find nothing. However, right as they prepare to leave, Hae-young recalls something Chi-soo said before he died and digs under the stairs in front of the house, discovering the skeletal remains of Lee Jae-han.
13	"The Real Assailant"	Kim Won-seok	Kim Eun-hee	March 4, 2016 (2016-03-04)
In 2001, after Lee Jae-han goes missing, he is framed for bribery by other officers. Soo-hyun is the only one who disbelieves this and resolves to find him, taking possession of his belongings including the notebook with his detailed case notes. In 1999, the Special Task Force squad returns to Seoul, and Jae-han prepares to submit his resignation due to his failure to prevent the framing of Park Sun-woo, but is convinced to stay by Soo-hyun. After remembering Hae-young's pleas over the walkie-talkie, Jae-han realizes the Park Hae-young he is talking to through time is the younger brother of the convicted Sun-woo. Tracking down the 9-year old Hae-young in Seoul, Jae-han discovers him neglected by his father and begging for food. Jae-han begins to watch over Hae-young, paying off a local restaurateur to feed the child. Hae-young grows up to become a troubled teenager and continues to eat at the same eatery. Remembering that the student witness who framed his brother claimed Sun-woo was chosen because he was poor and weak, Hae-young resolves to become strong, excelling in academics and eventually joining the police academy as it is the only high-end educational institution willing to overlook his history of delinquency. In 2015, the remains of Jae-han are brought in and positively identified, and a funeral is held for him. Hae-young finds the business card of the restaurant he used to eat at in Jae-han's belongings and questions the owner, discovering the truth of his secret benefactor. In 1999 and 2015, Hae-young and Jae-han talk across time on the walkie-talkie. Having changed his mind, Hae-young attempts to alter history by convincing Jae-han to give up on the Injoo case in order to prevent his death in 2001, but Jae-han refuses, promising that he will see the case to the end no matter the risk to himself. In 2015, while the police are investigating Hae-young for Chi-soo's murder with Kim Sung-bum having mysteriously vanished immediately after news of Jae-han's remains being found were leaked, Soo-hyun decides to have the cold case squad investigate the officially solved 1999 Injoo high school case, starting by searching for the surviving victim, Kang Hye-seung. Hae-young and Soo-hyun find the adult Hye-seung, who explains that the young Hye-seung was abused by her alcoholic father and Sun-woo was her only friend. When the rape allegation poem was released on the Internet by Lee Dong-jin, Hye-seung attempted to commit suicide by jumping off the high school's roof, but was stopped by Sun-woo. Later, Hye-seung was forced by her father to give the false testimony prepared by Bum-joo blaming Sun-woo for instigating the gang rape - the real culprit was a member of the student council.
14	"Not Now, But Maybe in the Past"	Kim Won-seok	Kim Eun-hee	March 5, 2016 (2016-03-05)
In 2015, Kang Hye-seung explains that the real perpetrator was a boy named Jang Tae-jin, the son of the president of Injoo Cement, a construction corporation that held power and influence in the town, and the nephew of Jang Young-chul, an influential congressman. When Hae-young offhandedly mentions that Sun-woo had committed suicide, Hye-seung is in disbelief, revealing that she had visited Sun-woo in prison shortly after he was sentenced, and the elder Park brother had consoled her and claimed he was going to start a new life once he had been released. As the statute of limitations on rape has long since expired and Hye-seung is unwilling to testify anyways as she now has a normal life and family, the cold case squad cannot touch Tae-jin for this crime, but Hae-young realizes it is still possible to change the past and prevent the deaths of both Sun-woo and Jae-han. Suspicious of Hae-young's cryptic statements, Soo-hyun goes to meet with him at his home that night. In 2000, with the Injoo high school case closed, Jae-han has now shifted his focus to investigating Bum-joo himself, and threatens the station chief in a heated confrontation. Bum-joo then returns the favor by threatening Jae-han's father, but Jae-han refuses to stop investigating. Later, while on a stakeout with Soo-hyun, Jae-han is asleep when Hae-young hails him on the walkie-talkie. Across time in 2000 and 2015, the younger and older versions of Soo-hyun simultaneously witness Hae-young attempting to reach Jae-han on the radio. Remembering what her younger self saw that night, the 2015 Soo-hyun finds Jae-han's walkie-talkie in Hae-young's apartment. In 2000, a few months after the resolution of the case, Sun-woo and the other students implicated are released from incarceration. Sun-woo finds and meets with Lee Dong-jin and forgives him, only requesting Hye-seung's red scarf. Out of guilt, Dong-jin decides to leave the country and study abroad, while instructing a fellow student to bring the red scarf to Sun-woo along with his apologies. While investigating Jae-han's broken walkie-talkie, Soo-hyun chances upon a detective who was Jae-han's partner before he joined the Special Task Force, who believes that Jae-han holds onto the walkie-talkie as a memento of Kim Won-kyung, his first love who was murdered by the Gyeonggi Nambu killer. In her jealousy, Soo-hyun sustains a minor ankle injury. Later, Jae-han receives a call from Sun-woo urgently requesting a meeting and claiming he has new evidence. Instead of going to meet with Sun-woo immediately, out of concern Jae-han accompanies Soo-hyun on a burglary bust, where he is stabbed and seriously wounded by the suspect - a distraught Soo-hyun later confesses her love for Jae-han in the ambulance. In 2015, Hae-young discovers Chi-soo had requested the case files of the Sun-woo suicide investigation before he died. He retraces Chi-soo's movements and discovers a blood sample in the Injoo hospital of Sun-woo's, proving that he had been sedated when he died. Soo-hyun confronts Hae-young with the walkie-talkie demanding to know the truth, and Hae-young comes clean, revealing how he had been in contact with Jae-han since the beginning of the Kim Yoon-jung case, using the communications to solve the cases, change the past, and even prevent the death of Soo-hyun that had occurred in a separate reality's 2015 in the Shin Yeo-jin kidnapping. While Soo-hyun is in disbelief, the walkie-talkie becomes active again, and Hae-young begs Jae-han to save Sun-woo, giving him the exact date of his death. In 2000, the wounded Jae-han leaves the hospital and races to save Sun-woo.
15	"Is It Really You?"	Kim Won-seok	Kim Eun-hee	March 11, 2016 (2016-03-11)
In 2000, while the wounded Jae-han drives to Injoo to meet with Sun-woo, Bum-joo arrives at Sun-woo's house, having received a tip from the Injoo police precinct. Sun-woo confesses to Bum-joo that the real perpetrator was Jang Tae-jin and reveals Hye-seung's red scarf. When Sun-woo explains that his motivation for clearing his name is to mend the rift between his mother and father, Bum-joo, who is protecting Tae-jin due to an imminent parliamentary meeting involving his uncle Jang Young-chul, decides to murder Sun-woo, lacing his drink with sedatives. Jae-han arrives too late and Sun-woo is found by Hae-young, with his murder being declared a suicide. Searching the house, Jae-han is unable to find the scarf, and learns from the officer at the precinct that Bum-joo had been tipped off to Sun-woo. Jae-han then violently confronts Bum-joo in his office. Defeated, Jae-han requests a transfer to a different precinct and gives a farewell to Soo-hyun. In 2015, Hae-young is arrested by the police for the murder of Ahn Chi-soo. Hae-young entrusts the walkie-talkie to Soo-hyun while he is taken into custody. The murder weapon, a knife, is found at Injoo hospital with Chi-soo's blood and Hae-young's fingerprints, while multiple eyewitnesses who had seen Hae-young after he discovered Chi-soo's body also implicate him for the crime. The next time the walkie-talkie becomes active again, Jae-han is eager to apologize to Hae-young for his failure, but it is instead the older Soo-hyun on the other end in 2015. Tearful, Soo-hyun attempts to prevent Jae-han's killing by telling him the exact date of his death and warning him to stay away from the location where it happened. In 2015, Soo-hyun meets with Hae-young in detention and demands to know if the past can be changed. She explains that she told Jae-han the date of his death, but Hae-young reveals that in the very first transmission he had ever received from Jae-han, it was on the day of his death and Jae-han had said that he had been warned not to go to that place - Jae-han knew of the danger and went anyways, discovering the body of Seo Hyung-joon had been a total coincidence. The cold case squad realize that Kim Sung-bum is also on the run now, due to Bum-joo needing someone to take the fall for the murder of Jae-han now that his remains had been found. Hae-young realizes it was Sung-bum who had attempted to contact him several times previously, and the squad retrieves his phone from evidence, where there is a message from Sung-bum requesting a meeting with Hae-young. Knowing that Sung-bum, an experienced criminal, would disappear if it is not Hae-young who arrives at the meeting location, Soo-hyun helps Hae-young to escape from police custody. They meet with Sung-bum, who reveals the truth of Jae-han's death: after transferring in 2000, Jae-han continued to investigate the Sun-woo suicide case, and somehow obtained forensic analysis implicating Bum-joo. By chance, the Kim Yoon-jung kidnapping case occurs, and Special Task Force reinforcements led by Bum-joo arrive and take over. While Jae-han is investigating that case, Bum-joo discovers the forensic evidence. Jae-han is then captured and tortured for information by Bum-joo, Sung-bum, and Chi-soo, but attempts to escape through a forest. Dying of a fatal stab wound, Jae-han makes his final radio transmission to Hae-young, telling him to convince his 1989 self and never give up as the past can be changed, before Chi-soo shoots and kills him. In 2015, Sung-bum is then killed by an agent sent by Bum-joo. The hitman then fires a shot at Soo-hyun; Hae-young leaps into the path of the bullet.
16	"If You Don't Give Up"	Kim Won-seok	Kim Eun-hee	March 12, 2016 (2016-03-12)
In 2015, while Hae-young is dying from the bullet, he and Soo-hyun desperately wait for the walkie-talkie to become active, but it does not. Hae-young later dies in the hospital. In 2000, days before Lee Jae-han dies, Kim Bum-joo discovers that Jae-han somehow obtained Kang Hye-seung's red scarf implicating Jang Tae-jin in the gang rape, and orders Chi Ahn-soo to follow him and retrieve it. Cha Soo-hyun transfers to the precinct and wants to speak with Jae-han, but he promises his investigation will be over soon and asks her to wait until then. In the original reality, Soo-hyun waits for over 15 years before Jae-han's remains are found in Sung-bum's country home. At the psychiatric hospital where Seo Hyung-joon's body was dumped, Jae-han delivers the very first radio transmission that Park Hae-young ever received, telling him the location of Hyung-joon's corpse, before he is knocked unconscious by Chi-soo. Jae-han later wakes up in an abandoned warehouse with Chi-soo, Bum-joo, and Sung-bum. It is revealed that after Bum-joo had murdered Park Sun-woo and retrieved the scarf, he had discarded it in the trash at a rest stop. Jae-han deduced this and managed to find the scarf at a landfill. Knowing that the police in Korea could not be trusted, he sent it to the United States for forensic analysis, and was later mailed the results, obtaining key evidence not only in the Injoo high school case, but also Bum-joo's blood linking him to Sun-woo's death. Bum-joo interrogates Jae-han as to who else knows about the scarf, but Jae-han breaks free of his bindings and flees into the forest. Right before Chi-soo fires the bullet that originally kills Jae-han, other officers appear on the scene and rescue him - this time, Jae-han had notified others where he was going. Jae-han and Soo-hyun later go on their first date to celebrate. In 2015, Hae-young suddenly wakes up after dying in the hospital, to find himself in his own apartment, but somewhat unfamiliar. He discovers nearly his entire life has been changed - in this reality, Jae-han had personally delivered the news to Hae-young's parents that Sun-woo had been posthumously exonerated, so his parents had never divorced. Hae-young tries to track down the cold case squad only to find that it no longer exists and the other two members, as well as the rest of the police, do not recognize him. Furthermore, in this new reality, Lee Jae-han is still missing and presumed dead, having disappeared at a later date in 2000: after tracking down Bum-joo, believing he had another copy of the floppy disk inadvertently stolen by Han Se-kyu and then again by Shin Da-hye implicating Jang Young-chul and many others in a corruption scheme, Jae-han and Bum-joo had been attacked. Bum-joo's corpse was later found at the scene, but not Jae-han's, who had escaped and then been framed for the murder. Remembering the notebook Jae-han had kept where he detailed the case notes for the four cases he and Hae-young had solved, Hae-young retrieves the notebook from evidence and finds an address written inside; the address is of his parents' house, who had received a letter from Jae-han containing the floppy disk and imploring Hae-young to continue the investigation. While searching the location where the envelope was mailed from, Hae-young chances upon Soo-hyun, who also remembers the original reality. Soo-hyun explains that after Jae-han's disappearance in 2000, she received a call that she believed was from him and tracked it down to the same location then. She had then searched for him for 15 years, never finding him alive or dead. However, a few days prior she received a strange text warning her to stay away from a certain treatment facility.They theorise either Jae-han is indirectly signalling them to come in there or someone from future is in contact with him. Holding out hope, Hae-young and Soo-hyun drive to that location, where it is revealed that Jae-han is still alive and is about to answer the active walkie-talkie thus confirming the latter's future theory to be true.

Original soundtrack

Signal OST
Soundtrack album by Various Artists
Released	March 15, 2016
Genre	Pop, K-pop, soundtrack
Producer	CJ E&M

Disc 1:

No.	Title	Artist	Length
1.	"회상" (Reminisce)	Jang Beom-june (Busker Busker)	4:27
2.	"떠나야할 그사람" (The One Who Will Leave)	INKII	4:06
3.	"나는 너를" (I Will Forget You)	Jung Cha-sik	4:24
4.	"길" (The Road)	Kim Yoon-ah	4:27
5.	"행복한 사람" (Happy Person)	Jo Dong-hee	3:34
6.	"꽃잎" (A Petal)	Leesa	3:16
7.	"꽃이 피면" (As Flower Blooms)	Lee Seung-yeol	3:38

Disc 2:

No.	Title	Artist	Length
1.	"Detective Lee Jae-han" (형사 이재한)	Various Artists
2.	"For Those Who Left Behind" (남겨진 이들을 위해)	Various Artists
3.	"Young Park Hae Young" (소년 박해영)	Various Artists
4.	"Conclusive Evidence" (결정적 증거)	Various Artists
5.	"Heartfelt Desire" (절실한 염원)	Various Artists
6.	"Send The Signal Desperately" (간절함이 보내온 신호)	Various Artists
7.	"New Clue" (새로운 단서)	Various Artists
8.	"Past Can Be Changed" (과거는 바뀔 수 있습니다)	Various Artists
9.	"Jae Han's Reason" (재한의 추리)	Various Artists
10.	"Pain That Can't Be Heal" (치유되지 않는 아픔)	Various Artists
11.	"Footsteps of Nostalgia" (그리움의 발자취)	Various Artists
12.	"Profiling" (프로파일링)	Various Artists
13.	"형기대의 하루"	Various Artists
14.	"Tracking" (추적)	Various Artists
15.	"Don't Give Up" (포기하지마)	Various Artists
16.	"Crime Scene" (범행현장)	Various Artists
17.	"A Series of Labyrinthine" (미궁의 연속)	Various Artists
18.	"Real Culprit" (진범)	Various Artists
19.	"Run Too, Today" (오늘도 달린다)	Various Artists
20.	"Only I Didn't Know" (혼자만 모른다)	Various Artists
21.	"Long-term Unsolved Case" (장기미제사건)	Various Artists
22.	"The Truth Revealed" (드러나는 진실)	Various Artists
23.	"Cool Air" (서늘한 공기)	Various Artists
24.	"Plot" (음모)	Various Artists
25.	"Suggestion" (암시)	Various Artists

Reception

Upon its premiere, the series attracted attention for weaving actual cases into its plot and attained high ratings. With its finale rating of 12.54%, it is one of the highest rated Korean dramas in cable television history. It received praise for its solid acting, tightly-constructed plot and detailed and sophisticated direction; and enjoyed success internationally in China and Japan.

The drama went on to win several awards from different award-giving bodies, including Best Drama, Best Screenplay for Kim Eun-hee, and Best Actress for Kim Hye-soo at the 52nd Baeksang Arts Awards, as well as another Best Actress award for Kim and the Daesang (Grand Prize for Television) for Cho Jin-woong at the tvN10 Awards. Cho also won the Daesang at the 1st Asia Artist Awards for his performance.

Ratings

Ep.	Original broadcast date	Title	Average audience share
			AGB Nielsen		TNmS
			Nationwide	Seoul	Nationwide
1	January 22, 2016	You're Doomed	5.415%	5.960%	6.4%
2	January 23, 2016	We Still Have a Chance	6.926%	7.343%	6.1%
3	January 29, 2016	We Can Prevent the Killings	8.239%	8.910%	8.5%
4	January 30, 2016	We Found the Killer	7.666%	8.601%	7.5%
5	February 5, 2016	Changing the Past Alters the Present	7.789%	7.568%	7.6%
6	February 6, 2016	Catch Him By All Means	7.077%	6.710%	7.3%
7	February 12, 2016	We Must Dig Deeper	8.612%	8.454%	9.0%
8	February 13, 2016	How Are Yoy Alive?	7.784%	7.913%	8.6%
9	February 19, 2016	Hongwon Dong Case	7.803%	7.672%	8.4%
10	February 20, 2016	Someone Has to Stop the Bad Guys	9.195%	8.945%	8.7%
11	February 26, 2016	The One Last Case	10.456%	11.915%	8.8%
12	February 27, 2016	The Case Was Manipulated	10.065%	10.865%	9.2%
13	March 4, 2016	The Real Assailant	9.667%	8.918%	9.7%
14	March 5, 2016	Not Now, But Maybe in the Past	11.120%	12.155%	10.6%
15	March 11, 2016	Is It Really You?	10.772%	12.145%	9.8%
16	March 12, 2016	If You Don't Give Up	12.544%	13.541%	12.8%
Average			8.821%	9.226%	8.7%
In this table, the blue numbers represent the lowest ratings and the red numbers represent the highest ratings. This drama airs on a cable channel/pay TV which normally has a relatively smaller audience compared to free-to-air TV/public broadcasters (KBS, SBS, MBC and EBS).

Awards and nominations

Year	Award	Category	Recipient	Result	Ref.
2016	52nd Baeksang Arts Awards	Best Drama	Signal	Won
		Best Director	Kim Won-seok	Nominated
		Best Actor	Cho Jin-woong	Nominated
		Best Actress	Kim Hye-soo	Won
		Best Screenplay	Kim Eun-hee	Won
	5th APAN Star Awards	Best Screenwriter		Won
		Top Excellence Award, Actor in a Miniseries	Cho Jin-woong	Won
		Top Excellence Award, Actress in a Miniseries	Kim Hye-soo	Nominated
	9th Korea Drama Awards	Best Drama	Signal	Nominated
		Best Production Director	Kim Won-seok	Nominated
		Best Screenplay	Kim Eun-hee	Nominated
	tvN10 Awards	Grand Prize (Daesang), Actor	Cho Jin-woong	Won
		Best Actor		Nominated
			Lee Je-hoon	Nominated
		Best Actress	Kim Hye-soo	Won
		PD's Choice Award	Lee Je-hoon	Won
		Scene Stealer Actor	Jang Hyun-sung	Nominated
		Best Content Award, Drama	Signal	Won
	1st Asia Artist Awards	Grand Prize (Daesang)	Cho Jin-woong	Won
			Kim Hye-soo	Nominated
		Best Celebrity Award, Actor	Lee Je-hoon	Nominated
	18th Mnet Asian Music Awards	Best OST	Jang Beom-jun	Nominated
	Korea Content Awards	Presidential Commendation Award	Kim Eun-hee	Won
2017	Brand of the Year Awards	Drama Writer of the Year		Won

Adaptations

• A Japanese remake starring Kentaro Sakaguchi aired from April 10 to June 12, 2018, on Fuji TV.
• On October 23, 2019, Unknown Number, a Chinese remake of Signal, started airing on Tencent Video.
• On January 20, 2023, there was also a Thai remake of Signal called 23:23, which aired on True4U.
• There is also an India remake of the series called Gyaarah Gyaarah (Web Series), which will stream in 2024 on Zee5.

See also

• Hwaseong serial murders
• Miryang gang rape

External links

• Official website (in Korean)
• Signal at IMDb