A group of 20 high school students partake in an elite class to prepare for a college exam. Among them are the rebellious Kang I-na, her best friend Yoon Myong-heo, and the class clown Kang Hyun, who harbors an affection for I-na. After several strange incidents, the class is interrupted by a TV showing of the top-ranking student, Min Hye-yeong, being trapped in a glass being slowly filled with water. A voice over the PA system announces that questions will be posed to the class; failure to answer them in time will lead to their death. It also warns them and their two teachers, Hwang Chang-wook and Choi So-yeong, against leaving the school. The class fails to solve the first question to save Hye-yeong.