Season 1

Kyung-min leaves a message to Jong-suk at a crime scene. Jina delivers the message to Jong-suk. Thus, Jong-suk and Kyung-min reunite through the message for the first time in twenty years.

Jong Suk helps Ji Na solve the case. They found out that Kyung Min lived with an alias for a certain amount of time Meanwhile, Kyung Min determines revenge against An Jeong Hee. who tormented him during his school days.

Kyung Min leaves a second message to Jong Suk at the murder scene. Jong Suk realizes that Kyung Min is revenging every perpetrator of school violence. And Jong Suk reminds a terrible memory from the past.

A special investigation team is launched and Jong Suk and Ji Na chase after Kyung Min. The team investigates the graduates of Shin Suk middle school. Ji Na meets the teacher and Jong Suk goes to Kang Min.

Jong-suk confesses that he also was one of the victims of school violence to Jina. And a video of Kang-min confined by Kyung-min arrives.

Jong-suk and Jina figure out the place where Kang-min is kidnapped. Kyung-min revives Kang-min's memory via quizzes. Kyung-min asks about Chul to Kang-min.

Jina got to know another key figure, Kim Chul. Jina asks Jong-suk about Chul, but he stays silent. So, Jina searches for the truth by herself.

Jong-suk figured out the next crime target and rushed to him. The third target sincerely apologizes to Jong-suk. Jina meets Chul's mother to investigate Chul.

Jina gets a step closer to the truth with the handwriting analysis of Chul's suicide note. Jong-suk reveals the identity of "Dog Park" as he looks for Kyung-min's next target.

Jina heads to the mansion to discover Chul's unperformed project. There she finds a note that Kyung-min left and confronts the truth from twenty years ago.

Jina detects the truth behind Chul's death. While Jong-suk's anxiety intensifies, Kyung-min comes into the police station.

Jong-suk and Jina doggedly track down Kyung-min. Jong-suk, Kyung-min, and other alumni from twenty years ago gather in one place.