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.

Woo-ram, the one with the highest votes so far, is about to lose his mind out of fear, and Kyung-jun, the one with the second highest votes, tries to take him down before midnight. Meanwhile, Yoon-seo is stuck on the second floor and finds a photo of the girl she saw in her dream.

Kyung-jun tries to control the rest of the survivors after he catches the second Mafia. He wants to get back at Jun-hee and the rest who voted for him instead of Woo-ram. Furthermore, Kyung-jun and his friends start to suspect Yoon-seo, who knows where to check the security camera footage.

Seung-bin is the next target who gets killed by the Mafia overnight. The survivors find him dead on the floor, and one of his friends is washing the bloody clothes in the toilet. Meanwhile, Yu-jun witnesses Kyung-jun and Jin-ha walking out of the building.

Uncovering yet another fatality, the students find themselves gripped by an encroaching sense of fear, with death looming ever closer. The distinction between Mafia and non-Mafia individuals now appears inconsequential, intensifying their apprehension. As a footprint and a message left on a wall emerge, the list of potential targets is narrowed down to two, heightening the emotional tension just before another vote is about to take place.

The deaths of Jin-ha and Kyung-jun, both innocent Citizens, heighten the students' sense of entrapment. The prevailing unease about the unreliability of seemingly plausible information forces them to prioritize unmasking the Mafia swiftly for their own survival. Meanwhile, So-mi, for some reason, begins to cast doubt on Na-hee, setting the stage for a sudden turn of events.

With Na-hee exposing her identity, everyone discovers that So-mi's the Mafia, placing Na-hee in danger with her police role now known. Holding herself responsible for her friends' deaths, Na-hee devises a plan to unveil the Mafia even after her potential death. Although clues surface, they are too ambiguous to conclusively identify the Mafia. Meanwhile, the focus shifts to Mi-na as the next target in the voting process.

Everyone corners Yeon-woo as the Mafia because he got wounded. He tries to defend himself, but he gets voted in the end. Da-bum feels relieved, and Jun-hee catches that moment. He confronts Da-bum and tells him not to kill Yoon Seo and Jung-won. Yeon-woo’s identity is revealed, and Yoon-seo finds out who the last Mafia is. Meanwhile, Da-bum wants to end the game to avoid being suspected.

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, and Yoon-seo is unexpectedly exposed to the shocking secret behind the game and the people involved in it.