Hae Young is a police lieutenant who is an expert criminal profiler. When he was young, his friend Yoon Jung was kidnapped and later found dead. The case remains unsolved to this day. Hae Young walks out of the station one day and hears someone talking to him. He searches and finds a walkie-talkie with the voice of detective Lee Jae Han, but Jae Han is from decades into the past. Cha Soo Hyun was a rookie during Jae Han's time, now an experienced police detective. Can they solve this mystery?