Overview
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.
| Actor | Age then | Age now | |
|---|---|---|---|
 | 15 | 32 | |
 | 19 | 35 | |
 | - | - | |
 | 38 | 54 | |
| - | - | |
 | 28 | 44 | |
| - | - | |
| - | - | |
| - | - | |
 | 19 | 35 |