After receiving an anonymous letter stating that someone would die, eight students at the elite boarding school, Susin High School, nicknamed "Prison High", decided to remain at school for the 8 days of winter break. To accompany them, a teacher is forced to remain. On their first night, a psychologist named Kim Yo Han takes refuge at the school after being involved in a car accident nearby. Due to the snowstorm, the roads are closed, and he is forced to spend the winter holiday with them. However, at a time when everyone else is celebrating Christmas Eve, the students realize that the anonymous letters they each received were not the result of a harmless prank; and that there is a murderer in their midst.