Fifteen years ago, Park Hae Yeong witnessed the kidnapping of a young girl on her way home from school. Days later, she was found dead, and the case remained unsolved. The experience left Hae Yeong with a deep mistrust of the police.

Now a police officer and criminal profiler, Hae Yeong discovers a walkie-talkie that connects him to Detective Lee Jae Han in the past. Together with detective Cha Soo Hyun, they work across time to solve long-forgotten cold cases.