Signal

Hae-young finds an old walkie-talkie at the police station. Detective Lee Jae-han from the other side seems to hold clues to a 15-year-old kidnapping.

The statute of limitations is lifted after the suspect's arrest. Hae-young joins Soo-hyun's team for the newly established cold case squad.

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.

Reality changes after Hae-young exchanges information with Jae-han. Hae-young tries to work together with Jae-han in 1989 to prevent the next murder.

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 ninth victim is Won-kyung, the woman Jae-han loves. A driver turns himself in, but Soo-hyun and Jae-han determine that his son is the real killer.

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 starts to investigate Jae-han's disappearance. Jae-han's former colleagues get suspicious about seeing Hae-young digging into the past.

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

Seeking vengeance for the events that led to the death of his daughter, Kyung-tae commits a kidnapping after getting out of prison for burglary.

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

After Su-hyun's death, Hae-young considers whether to keep the walkie-talkie. In the past, Jae-han uncovers the truth behind a serial robbery case.

Soo-hyun and Hae-young notice a connection between the burglary case and Da-hye, the woman who killed herself twenty years ago.

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.

Hae-young suspects Chi-soo may be involved in Jae-han's disappearance when he unexpectedly gets hold of one of Jae-han's belongings.

The culprit of the Hongwon-dong serial murder case is uncovered. Meanwhile, Jae-han learns that Su-hyun and Hae-young are on the same team in 2015.

Hae-young and Soo-hyun arrest Jin-woo after coming to the conclusion that he is the killer. Hae-young asks Jae-han about the Inju rape case.

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.

Hae-young becomes a suspect for the murder of Chi-soo. Jae-han realizes who the principal offender of the attack on Hye-seung is.

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.

Soo-hyun and the others mourn after confirming that the remains belong to Jae-han. Hae-young learns that Jae-han used to look after him in the past.

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.

Hae-young goes to Inju and interrogates the man who accused his brother, learning about Chi-soo. Meanwhile, Jae-han gets a call from Seon-woo.

Jae-han rushes to Inju to save Seon-woo. Hae-young is arrested for Chi-soo's murder, while Su-hyun gets closer to the walkie-talkie's secret.