The film is set in a high school, where an elite group of twenty students—including rebellious heroine Kang Yi-na, her timid best friend Yoon Myong-hyo, and her would-be boyfriend Kang Hyeon—are taking a special class for their college entrance exam. After Kang Hyeon is nearly strangled and another student throttled in the restroom, the classroom TV screen switches to an image of top-ranking student Hye-yeong trapped inside a fish tank that is slowly filling with water. A disembodied voice announces that her life depends on the exam questions he will set for them, and that a student will die for every question the class gets wrong. Trapped with the students are head teacher Hwang Chan-wook and English teacher Choi So-yeong. Yi-na realises that the students are being killed in order of their rank in the class, and she is ranked fifth.