When class 2-3 arrives at a retreat center, the rest of the classes are nowhere to be found. Their homeroom teacher goes out to look for the missing classes, and they are left alone in the building. Meanwhile, Yoon-seo and all of her classmates get a message out of the blue. With the rules of the game, each one of them receives a role, and the game begins.

As morning dawns, the kids try to figure out what this game is actually about. They realize that they can’t leave the boundary indicated in a white line all around the retreat center. Some kids try going up the trail to see if there’s a break in the line. However, things only seem to get worse as they start testing the rules and doubting each other more and more.

The Mafia Game nears its conclusion as more classmates are executed. Da-bum continues to threaten the remaining students’ lives. Meanwhile, Jung-won faces a dilemma with her life at stake.