Signal

Profiler Park Hae-young hears a man calling his name through a discarded two-way radio. The voice starts talking about an unsolved crime.

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.

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.

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.

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

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

Jae-han arrests Se-kyu for the serial burglary case, but the culprit is freed after serving a short sentence; Kyung-tae still commits murder.

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

Hae-young learns that Chi-soo and Jae-han met in 1999 during a case in Inju. After serving time for the crime, Hae-young's brother had killed himself.

The bodies discovered in the mountains appear to be more victims of the 1997 serial murder case. Jae-han learns that Soo-hyun became team leader.

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

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.

Hae-young realizes the possibility that his brother could have been murdered after learning from Hye-seung that Sun-woo was not guilty.

Hae-young is arrested for the murder of Chi-soo. Soo-hyun hears Jae-han apologizing over the radio for not being able to prevent Sun-woo's death.