Signal

A criminal profiler finds an old walkie-talkie that mysteriously connects him to a detective living fifteen years in the past. With time itself as their only link, the two men begin working together to solve unsolved crimes before they happen while veteran detective Cha Soo-hyun helps piece together cases that have haunted the police for years. But every change to the past creates new consequences in the present, forcing them to confront a chilling question: how much can they change before they lose the people and lives they were trying to save?