A certain friend recommended it to me.This drama has good plot. Actors in this drama are Kim Hye Soo (The Thieves, Coin Locker Girl), Lee Ji Hoon (Architecture 101), Jo Jin Woong (Assasination, Entourage). It is about criminal profiler Lee Ji Hoon can talk to detective in past time with walkie talkie. from this communication, they can resolve crime mistery in the past time. Some of its episode are based on true story so it make me more curious about what happens actually. Signal become one of korean drama that airing in cable TV (TvN) with second highest rating, beating non cable TV. So for, this drama is really a must watched korean drama.