Signal

Profiler Park Hae-young receives an unexpected call on a discarded two-way radio, where a man mentions an unsolved crime, drawing him into a mysterious investigation.

With the statute of limitations removed following the suspect's arrest, Hae-young becomes part of Soo-hyun's team as they form a new cold case unit to revisit unsolved crimes.

After Hae-young shares information with Jae-han, reality begins to shift. Determined to stop a future murder, Hae-young collaborates with Jae-han in 1989 to alter the course of events.

Jae-han and Soo-hyun investigate the murder of Won-kyung, the woman Jae-han is in love with, as the ninth victim. A driver confesses to the crime, but the detectives uncover evidence pointing to his son as the true killer.

Hae-young begins looking into Jae-han's disappearance, prompting unease among Jae-han's former coworkers who notice her inquiries into old matters.

Kyung-tae, seeking revenge for the death of his daughter, kidnaps someone after being released from prison for burglary.

Jae-han successfully arrests Se-kyu for a series of burglaries, but Se-kyu is released after serving a brief sentence. Meanwhile, Kyung-tae continues his violent actions by committing murder.

Soo-hyun and Hae-young uncover a potential link between a recent burglary and Da-hye, a woman who died by suicide two decades earlier.

Hae-young discovers that Chi-soo and Jae-han crossed paths in 1999 during an investigation in Inju. Following his brother's imprisonment for the crime, he later committed suicide.

Bodies found in the mountains are suspected to be additional victims of a 1997 serial killer case. Jae-han discovers that Soo-hyun has been promoted to team leader.

Hae-young and Soo-hyun take Jin-woo into custody, believing him to be responsible for the murders. Meanwhile, Hae-young inquires about the Inju rape case with Jae-han.

Hae-young is implicated in Chi-soo's murder, while Jae-han uncovers the identity of the person responsible for the attack on Hye-seung.

Soo-hyun and the group grieve upon identifying the remains as those of Jae-han. Hae-young discovers that Jae-han had once cared for him.

Hae-young begins to suspect that his brother may have been murdered when he learns from Hye-seung that Sun-woo was not responsible for his death.

Hae-young is taken into custody for Chi-soo's murder, while Soo-hyun listens to Jae-han express regret on the radio for failing to prevent Sun-woo's death.

Hae-young is critically injured after being shot during a meeting with Sung-bum, while in 2000, Jae-han is captured by Bum-joo.