Original: 터널
A police detective from 1986 travels in time to the present to save his daughter.
In the town of Hwayang, series of murders that took young women's lives away is making everyone feel anxious. Serious Crimes Unit of Hwayang Police Station works day and night to catch the culprit behind the incidents, but for months, they make no progress. At last, Kwang Ho makes a breakthrough and tracks down the murderer behind the murders.
Detective Park Kwang Ho is shocked to find himself transported 30 years forward in time to December 2016, where he assumes the identity of another police officer bearing his name and starts working on a case with a new partner.
Detective Park Kwang Ho, after realizing that he's unable to go back to 1986, is convinced he was sent to the future to solve the 30-year-old serial murder case, but clashes with his coworkers over a new unidentified victim.
Team Leader Sung Shik figures out who Park Kwang Ho's real identity, and Kwang Ho asks him for help finding Yeon Sook. A burglary case leads to murder, and a criminal psychologist, Professor Shin, is brought in for an unusual request.
Park Kwang Ho and Kim Seon Jae investigate a murder in a roadside rest area that brings up memories for Professor Shin Seon Jae. Park Kwang Ho learns that the officer whose identity he assumed is missing.
A stolen identity case causes Team Leader Sung Shik to worry that Park Kwang Ho will be found out. Professor Shin looks into the Jung Ho Young case. Kwang Ho tries again, and in vain, to go back to his time.
Kim Seon Jae, believing Kwang Ho to be an identity thief and murderer, offers to help him investigate the other Kwang Ho, who died under mysterious circumstances. Meanwhile, the investigation into the serial killer's whereabouts continues.
Park Kwang Ho and Kim Seon Jae, who found a new respect towards each other, agree to investigate the Jung Ho Young case together. The police team tracks down an arsonist.
With Jung Ho Young attracting the public's attention and becoming bolder, the entire police station is ordered onto his case. Park Kwang Ho discovers what happened to his wife after he disappeared 30 years ago.
While the police scramble to pin down Jung Ho Young's location, Pr. Shin, being the only one who believes that he isn't responsible for all of the murders, decides to take action. Kwang Ho discovers who Pr. Shin is.
While searching for more evidence to convict Jung Ho Young, the police realize that they dealing with more than one killer. Kwang Ho cannot bring himself to tell Pr. Shin who he really is.
The police turn their attention to the second murderer. Kwang Ho begins to suspect Professor Mok, who has figured out who Kwang Ho really is. Pr. Mok lures Kwang Ho into a confrontation.
After jumping back to 1986 and reuniting with Yeon Sook, Kwang Ho tries to locate the second killer before he kills victim #5. Meanwhile, in 2017, the team's and Pr. Shin's search for Kwang Ho reveal that he was on a lead.
The police arrest Mok Jin Woo, hoping to get a confession before the 48 hours to produce evidence are up. Pr. Shin investigates his past to find his motives, and she explores her new relationship with her father.
While Park Kwang Ho and Kim Seon Jae are searching frantically for Pr. Shin, she stalls for time after being kidnapped by Mok Jin Woo, hoping that the police will be able to track him before he kills her.
Kim Seon Jae struggles to hold onto his composure as the police once again try to force Mok Jin Woo to confess. When all is over and after saying goodbye to his adult daughter, Kwong Ho goes back to 1986. All loose ties are connected.
