Hwang Kyung-Min (Kim Dong-Wook) was traumatized by school violence 20 years ago, but he now hides those feelings. He appears to be a happy person, enjoying life with his wife. An unexpected case causes his hidden emotions to reappear. Meanwhile, Detective Jung Jong-Suk (Kim Sung-Kyu) receives a mysterious message from his friend he knew 20 years ago. That leads him to remember the school violence that took place back then. Chasing the mysterious message, he tries to stop more murders with the help of Detective Kang Jin-A (Chae Jung-An). She is a detective with charisma and enthusiastic about her work.