After receiving an anonymous letter stating that someone would die, eight students at the elite boarding school decided to remain at school for the 8 days of winter break. To accompany them, a teacher is forced to remain.
Susin High School, nicknamed "Prison High," is an elite 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 any activities outside of studying. It is in this atmosphere that seven students and a teacher remain at school for the winter break, as they all received anonymous letters, joined by Kim Yo Han, a psychiatrist who was forced to take shelter with them after he was involved in a car accident nearby. At a time when everyone else is celebrating Christmas Eve, the students realize that there is a murderer in their midst. A question lies unspoken: Are monsters created, or are humans born monsters?
