On the eve of the dreaded university entrance exams, the lives of 24 of one high school's smartest and brightest students are at stake. A television monitor broadcasts a terrified Hae-yeong, the top student at the high school, tied up and seated on a chair inside an aquarium that slowly begins to fill with water, locked inside a classroom with no way out. A voice from the monitor announces that if the 24 do not complete the exam within the allotted time, Hae-yeong will die, as will everyone else imprisoned inside the classroom. As a result, the examination that will determine whether they live or die begins.