Signal

Haunted by a classmate's kidnapping 15 years ago, Park Hae Young becomes a police profiler who's lost faith in the police force. This all changes when he stumbles upon an old radio that's receiving transmissions from the past.

Solving the kidnapping draws Hae-young into the Cold Case Task Force with Su-hyun, while Jae-han from 2000 tells Hae-young something mysterious.

After the Kim Yoon Jung kidnapping case, the Chief of police creates a cold case squad. Their first case is the infamous Gyeonggi Nambu serial murders.

During the investigation of the serial murders, Park Hae Young discovers that the radio transmissions are actually connecting him to the past.

Hae-young is shaken after realizing their walkie-talkie conversations have changed the past. Meanwhile, Jae-han in 1989 draws suspicion because of it.

The cold case squad figures out who the serial killer is, but not before the wrong suspect turns himself in.

Despite opposition from superiors, the Task Force investigates the South Gyeonggi Serial Murder case and uncovers the suspect in a 26-year-old crime.

Hae-young gets closer to the truth behind Jae-han's disappearance. Meanwhile, police are in an uproar over a mysterious kidnapping that night.

Park Hae Young begins asking questions about Lee Jae Han. Meanwhile, the police force in Lee Jan Han's time struggles with a string of larceny cases.

Hae-young and the police locate the kidnapper and the victim. Jae-han continues trying to fix a past and future now dangerously intertwined.

Park Hae Young figures out that Oh Kyung Tae's true objective is the father of the girl he kidnaps.

Lee Jae Han catches the real culprit behind the burglaries, unwittingly changing the past for the worse again.

The cold case squad finds Shin Da Hye, who has been living under the murder victim's identity.

After finding Da-hye's fingerprint, the Task Force traces her whereabouts. In the past, Jae-han unravels the truth behind the 1995 serial robbery.

Park Hae Young uncovers another layer to Lee Jae Han's disappearance. An old demon from Cha Soo Hyun's past resurfaces.

The Cold Case Squad is assigned to the serial murder, but the last the victims isn't identified. Cha Soo Hyun revisits a past trauma.

The Cold Case Squad catches the Hongwon-dong serial killer, but Lee Jae Han's determination changes the past again.

Hae-young asks Jae-han to reveal the truth about a case in Inju tied to his brother. Jae-han then goes to Inju to investigate a sexual assault case.

Park Hae Young becomes the prime suspect for the Section Chief's murder. Meanwhile, Lee Jae Han must find out who this "one" is before it's too late.

The police suspect Hae-young assaulted Chi-soo, and Su-hyun is shocked to learn that Chi-soo killed Jae-han. Jae-han investigates the Inju students.

Su-hyun anxiously awaits DNA results to see if the remains they found match Jae-han, while Jae-han learns Hae-young is Seon-woo's younger brother.

The Cold Case Squad secretly reinvestigates the Injoo gang rape case. Lee Jae Han finds out the connection Park Hae Young has to the case.

Following the trail the Section Chief left, Park Hae Young uncovers the truth behind his brother's death. Meanwhile, Cha Soo Hyun finds Lee Jae Han's radio in Hae Young's possession.

Park Hae Young is framed and arrested for the murder of the Section Chief. Cha Soo Hyun sets out to save Lee Jae Han and to find the one witness who can bring down their enemy.