Two friends in the past were victims of serious school bulling. One could not resist his trauma, and becomes a serial killer, walking in the path of revenge. Another one tries his best to overcome the trauma and becomes a police detective. He now faces tragic fate to risk his life to capture the serial killer.
E1 ∙ Prelude of Revenge
Kyung Min leaves a message for Jong Suk at a crime scene. Ji Na delivers the message to Jong Suk. Thus, Jong Suk and Kyung Min reunite through the message for the first time in twenty years.
E2 ∙ The First Revenge
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.
E3 ∙ Prelude of Tracking
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.
E4 ∙ Start of Tracking
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.
E5 ∙ Confession
Jong Suk confesses that he also was one of the victims of school violence to Ji Na. And a video of Kang Min confined by Kyung Min arrives.
E6 ∙ The Second Revenge
Jong Suk and Ji Na 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.
E7 ∙ Another Friend
Ji Na got to know another key figure, Kim Chul. Ji Na asks Jong Suk about Chul, but he stays silent. So, Ji Na searches for the truth by herself.
E8 ∙ The Third Target
Jong Suk figured out the next crime target and rushed to him. The third target sincerely apologizes to Jong Suk. Ji Na meets Chul's mother to investigate Chul.
E9 ∙ The Fourth Target
Ji Na 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.
E10 ∙ The King of Pigs
Ji Na 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.
E11 ∙ Past Is Present
Ji Na detects the truth behind Chul's death. While Jong Suk's anxiety intensifies, Kyung Min comes into the police station.
E12 ∙ The Last Target
Jong Suk and Jina doggedly track down Kyung Min. Jong Suk, Kyung Min, and other alumni from twenty years ago gather in one place.
