In a way, this drama reminds of Signal, but only with the time travel part. Otherwise, it is another fascinating drama by OCN based on the Hwaseong Murders. To give a little background, the Hwaseong Murders is a famous case that was never solved in S. Korea. Several women ranging from ages 14- 70 were strangled by their own clothing by a serial killer. Tunnel features a homicide detective in the 1980s who is in the middle of investigating a series of murders where the victim is strangled by her own pantyhose and the ankles are dotted in the order they were killed. After several false leads, detective Gwang-Ho manages to catch a glimpse of a mysterious man in a tunnel. While running after said man, he is knocked out on the head with a rock by the man. When he wake up and tries to get out of the tunnel, he ends up walking into another world. This world is the same as his, but 30 years in the future. He is extremely confused as to what is going on and tries to make sense of it by going to his police station. He pretends to be an officer with the same name, but his experience gives him away. Coincidentally, a junior officer of his has now become chief at the same station. His junior officer recognizes his friend that had disappeared 30 years ago. In the past, his body is never found and his team is disbanded after the tragic event. Together, in the future, they try to make sense of what has happened, where Gwang-Ho’s wife is, and how to get back to his time. Of course, adapting to the new traditions at the police station is difficult. He ends up clashing with Sun Jae, an extremely smart detective who is also quite abrasive. Joining the bunch is Jae-Yi, a criminal psychologist who lends her expertise to the homicide unit.