Signal (Korean Drama) (2015)

Fifteen years ago, a female child was kidnapped on the way from school. At that time, Park Hae Yeong, who was an elementary school student, witnessed the girl being taken away. A few days later, the girl was found dead, and detectives couldn't 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 So Hyun then set out on a journey to solve cold cases.