In Boseok Boys' High School, the students of Grade 12 – Class 4 are trying to come into terms with the possibility of a ghost lurking in their homeroom after discovering that their class of 21 is supposed to have 20 students only. A series of spooky events push the students to suspect each other while trying to solving the mystery of their class' resident ghost, who might be simply posing as one of their classmates.