Signal

Signal (Korean: 시그널; RR: Sigeuneol) is a 2016 South Korean fantasy police procedural television series starring Lee Je-hoon, Kim Hye-soo and Cho Jin-woong. It aired on tvN from January 22 to March 12, 2016 on Fridays and Saturdays at 20:30 KST for 16 episodes.^[2][3][4] The series was inspired by real-life criminal incidents in Korea,^[5] including the Hwaseong serial murders.^[6][7]

A walkie talkie allows a detective in the year 1989 to communicate with a cold case profiler from 2015. With the power of fore- and hindsight, the two not only solve crimes but prevent them from ever taking place.

A criminal profiler Park Hae-young (Lee Je-hoon), solves a kidnapping case that ended up with a falsely accused who apparently disappeared after the crime, with a mysterious walkie-talkie he picks up. The success of this case triggers the formation of a long-term cold case team, led by Detective Cha Soo-hyun (Kim Hye-soo), who has been searching her long-lost mentor Lee Jae-han (Cho Jin-woong), for the past fifteen years. With the help of Detective Lee Jae-han, the person at the other end of the walkie-talkie, Park Hae-young solves other cold cases that have remained unsolved for years, while helping Lee Jae-han help solve some cases in the past. Grave unintended sequences follow.

Source : Wikipedia