While investigating a serial murder case, Han Tae Joo gets into an incident. When he wakes up, he finds himself back in 1987. To get back to his present world, he must solve the murder case.

Moo Jin-Hyuk (Jang Hyuk) was a popular detective who solved major cases, but after his wife was murdered by a serial killer his life spiraled down. He is filled with guilt that he was not able to protect his wife and he secretly looks for her killer. Meanwhile, Kang Kwon-Joo (Lee Ha-Na) graduated at police academy with top marks. She began to work at an emergency 112 call center. Working there, a brutal murder case takes place and her father/police sergeant went to the crime scene. Her father is killed there and Kwon-Joo hears what happened over the phone. Afterwards, she goes to the U.S. to study and comes back to Korea as a professional voice profiler. Detective Jin-Hyuk and Kang Kwon-Joo solve cases together. Chasing a serial killer responsible for the deaths of their family member.

An explosion took place in the past. That explosion led to a serial murder case known as the Chimera case. 35 years later, a similar explosion takes place. Cha Jae-Hwan (Park Hae-Soo) is an enthusiastic detective. When Eugene (Soo-Hyun) was little, she was adopted by parents in the United States. She once worked for the FBI and she now works as a criminal profiler. When Lee Joong-Yeop (Lee Hee-Jun) was little, he was adopted by parents in England. He became a surgeon and returned to South Korea, but he gets involved in a mysterious case. Cha Jae-Hwan, Eugene and Lee Joong-Yeop have different reasons to chase the truth of the Chimera case.

The drama is about a highly successful lawyer who falls from grace because of one case. He becomes a fixed term teacher at a high school that is tied to that case in order to regain his honor. Though he goes into the classroom for selfish reasons, he begins to see the crimes that take place and he takes matters into his own hands to help protect victims from their peers. The drama will shine a spotlight on the reality of teenagers facing little to no repercussions after committing a crime because they are teenagers, providing no protection for those who become a victim to these crimes.

Fifteen years ago, a female child was kidnapped on the way from school. At that time, Park Hae Yeong, who was an elementary school student, witnessed the girl being taken away. A few days later, the girl was found dead, and detectives couldn't find the culprit. As time went by, Hae Yeong started distrusting the police. Fifteen years later, Hae Yeong is now a police officer and criminal profiler. One day, he finds a walkie-talkie that allows him to get in contact with Detective Lee Jae Han who is in the past. Police officers Hae Yeong, Jae Han and Cha So Hyun then set out on a journey to solve cold cases.

Kang Yo Han (Ji Seong) is a judge. He has an aristocratic appearance and elegant taste. He punishes corrupt and greedy men with power. Kang Yo Han a secret. Jung Seon Ah (Kim Min-Jung) is an executive director of the Social Responsibility Foundation. She is smart and beautiful. She is also Kang Yo Han's enemy. Jung Seon Ah is involved with important people from various social circles like politics and economics. She has the power to control the country by using her relationships. Kim Ga On (Park Jin-young) is a rookie judge. He has experienced difficult times since his parents died when he was a child, but he was able to become a judge after expending great effort. Yoon Soo Hyeon (Park Gyuyoung) is a detective. She is Kim Ga On's long time friend and she chases after Kang Yo Han's secret.

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