Fifteen years ago, a young girl was kidnapped on the way from school, and Park Hae Yeong, who was an elementary school student at that time, witnessed the crime. A few days later, the girl was found dead, and the police were not able to find the culprit. As time went by, Hae Yeong started distrusting the police. Fifteen years later, Hae Yeong is now a police officer and criminal profiler. One day, he finds a walkie-talkie that allows him to get in contact with Detective Lee Jae Han who is in the past. Police officers Hae Yeong, Jae Han, and Cha Soo Hyun then set out on a journey to solve cold cases.