A walkie-talkie in the present somehow picks up signals from a detective in the past, Lee Jae Han (Jo Jin Woong). Police profiler Park Hae Young (Lee Je Hoon) finds this walkie-talkie and uses it to help solve cold cases. Detective Cha Soo Hyun (Kim Hye Soo), who was once a rookie at Lee Jae Han’s station, leads the cold-case team. Thanks to Park Hae Young (and Lee Jae Han), the team solves infamous cold cases. But communicating with the past has consequences.