Detectives from the present and a detective from the past communicate via walkie-talkie to solve a long-time unsolved case.
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.
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.
The cold case squad figures out who the serial killer is, but not before the wrong suspect turns himself in.
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 case
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.
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.
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 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.
Lee Jae Han is set on bringing down Kim Bum Joo, despite knowing that he's going to die, but a certain transmission is about to make all the difference.
