At an exclusive high school in Seoul, examination marks matter and so do parents' social status. To prepare for a visit from students from England, two popular teachers, Mr. Kim and Ms. Choi, gather top pupils for a Saturday study session. Things go awry: one by one, students disappear, dying in grisly ways, sometimes in view of the others via closed circuit TV. A voice on the P.A. system warns of death if anyone tries to leave. The voice also asks questions, and Mr. Kim leads the students in trying to solve them, hoping to save a student. Will the ordeal come to an end, and is the cause rational or supernatural? What's the pattern?