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 kids feel guilty and sad about the ones who have died, and this pushes them to drift apart and blame one another. Kyung-jun, the class bully, continues to pick on others and use them to test the rules of the game. Meanwhile, Class President Jun-hee has an idea to refuse to vote for anyone in the game that night.

As Jun-hee’s plans to refuse to vote backfires, a bloodbath sweeps the assembly hall. The remaining kids are as divided as ever in the aftermath of losing so many kids in one night. Meanwhile, Yoon-seo is suspicious of the visions she keeps having. She decides to scope out the mysterious tunnel that she found.

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.