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, joined by Kim Yo Han (Kim Sang Kyung), 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 the anonymous letters they each received were not the result of a harmless prank; there was a murderer in their midst. A question lies unspoken: Are monsters created, or are humans born monsters?