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.
| Directing | Sang-yong Lee | Director |
| Writing | Shin Jai-ho | Writer |
| Art | Kim Jin-chul | Production Design |
| Editing | Kyung Min-ho | Editor |
| Sound | Lee Sang-wook | Production Sound Mixer |
| Art | Jeon Jin-won | Production Design |
| Visual Effects | Kim Byeong-gi | Special Effects Supervisor |
