A criminal profiler Park Hae Young , solves a kidnapping case that ended up with a falsely accused who apparently disappeared after the crime, with a mysterious walkie-talkie he picks up. The success of this case triggers the formation of a long-term cold case team, led by Detective Cha Soo-Hyun, who has been searching her long-lost mentor Lee Jae-Han,
for the past fifteen years. With the help of Detective Lee Jae-Han, the person at the other end of the walkie-talkie, Park Hae-young solves other cold cases that have remained unsolved for years, while helping Lee Jae-Han help solve some cases in the past. Grave unintended sequences follow.