In 1986, Detective Park Gwang Ho (Choi Jin Hyuk) is desperately chasing a serial killer. While pursuing the killer, he enters a tunnel. Upon emerging on the other side, he finds himself in 2017. The serial killer has resumed the murders he started 30 years ago. Detective Park Gwang Ho works with Detective Kim Sun Jae (Yoon Hyun Min) and Criminal Psychology Professor Shin Jae Yi (Lee Yoo Young) to catch the killer.
