Two friends meet up and recall their experiences as victims of school bullying when they receive a message from a friend from 20 years ago and mysterious serial killings begin to occur. Kyung Min lives with the trauma of the school violence he experienced twenty years ago and an unexpected case causes his suppressed emotions to reappear. Meanwhile, Detective Jong Suk receives a mysterious message sent from a friend from 20 years ago which reminds him of the school violence that took place back then. Chasing the mysterious message, he tries to put a stop to the murders. Through these characters with tragic fates, the drama touches on the origin of school violence and the bullying that pervades modern society.