Signal

In present-day 2015, criminal profiler Park Hae-young (Lee Je-hoon) finds a mysterious discarded walkie talkie; inexplicably, the person at the other end, Detective Jae-han (Cho Jin-woong), is from the past, and addresses him familiarly, even though Hae-young has never met him. Hae-young is able to solve a 15-year-old kidnapping and murder case involving a former classmate from his childhood with Jae-han's help. The success of this case triggers the formation of a cold case team, led by Detective Cha Soo-hyun (Kim Hye-soo), who has been searching for Jae-han, her long-lost mentor and unrequited love, for the past 15 years. With Jae-han's help from the 1990s, Hae-young solves other cold cases that have remained unsolved for years and is drawn into the mystery surrounding Jae-han's disappearance in the year 2000. As they try to fix the past and catch the criminals, unintended consequences follow in the present.