Signal (2016)

The 2016 Korean original, and the cleverest twist on the idea here. In 2015, profiler Park Hae-young picks up a discarded walkie-talkie and finds himself talking to a detective working in 2000, and together they chip away at old cases that police in the present never closed. Several of its storylines were inspired by real Korean crimes, including the Hwaseong serial murders, which gives the fantasy premise real weight. It won Best Drama at the Baeksang Arts Awards, and Kim Hye-soo won Best Actress there as the head of the cold case team. At 16 episodes it is a long haul, but the payoffs land.