During fourth period, high schooler Da-jeong finds her classmate Tae-gyu dead in a classroom and Jeong-hun standing in front of him with blood on his clothes. In order to stop Jeong-hun from becoming the prime suspect, Da-jeong must help him find the real murderer, who is still inside the school building, under 40 minutes before period four ends.

Sung geun, a physical education teacher, is at risk of losing his job when the school decides it needs an English teacher instead of a PE teacher. Therefore, he starts to devise a plan to change his job as an English teacher.