Signal

Police officer Park Hae Young, haunted by a classmate's murder 15 years prior, becomes disillusioned in law enforcement; his outlook changes upon discovering a mysterious walkie-talkie that picks up a strange transmission.

The chief of police establishes a cold case team after the Kim Yoon Jung kidnapping and assigns them to investigate the infamous Gyeonggi Nambu serial murders.

As he works to solve the serial murders, Park Hae Young realizes the true source of the radio transmissions originates in the past.

As the cold case squad closes in on the serial killer's identity, a man turns himself in who cannot be the true suspect.

Park Hae Young wants to know more about Lee Jae Han; a series of larceny cases occupy the police department in Lee Jan Han's era.

Park Hae Young realises that kidnapper Oh Kyung Tae has designs on his hostage's father, forcing him to take action.