During fourth period, high schooler Da-jeong finds his 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.
![4th-Period-Mystery-2009](http://167.86.120.57/wp-content/uploads/2018/04/4th-Period-Mystery-2009.jpg)