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.
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 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.
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.
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.
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.
