Susin High School, nicknamed "Prison High," is an elite boarding school attended by the top 1% of students in the country. Their stellar marks are the result of constant pressure and a strict punishment system, to the point where students avoid from any activities outside of studying and even study on weekends. Furthermore, their only vacation days are the 8 days from Christmas Eve to the first day of the New Year. read moreDuring this time, the students and faculty leave the school and the school becomes empty.
However, seven students decided to remain at school on winter break after receiving an anonymous letters stating that someone will die. To accompany them, a teacher is forced to remain at school with them. But on their first night, a psychologist called Kim Yo Han joins them after being involved in a car accident nearby. In addition, because of 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 was not the result of a harmless prank; and there is a murderer in their midst.