In 1986, Detective Park Gwang-Ho (Choi Jin-Hyuk) desperately tries to catch a serial killer. He chases after the serial killer and goes through a tunnel. On the other side of the tunnel, Detective Park Gwang-Ho finds himself in the year 2017. The serial killer has resumed the killings that began 30 years ago. Detective Park Gwang-Ho works with Detective Kim Sun-Jae (Yoon Hyun-Min) and Professor of Criminal Psychology Shin Jae-Yi (Lee Yoo-Young) to catch the killer.