Students on a school retreat find themselves alone when their classmates and teachers vanish. They receive a mysterious message assigning roles for a strange game that seems to trap them in the building.

When children at a retreat wake up, they find themselves trapped within a mysterious boundary. Attempting to escape only heightens tensions as the kids begin questioning the rules and distrusting each other.

Guilt over deaths causes kids to drift apart and blame each other. Kyung Jun bullies and tests game rules. Jun Hee plans to refuse voting that night.

A bloody massacre occurs in the assembly hall after Jun Hee's plans to boycott the vote backfire. The surviving students remain divided after the traumatic losses. Yoon Seo investigates visions by exploring a mysterious tunnel.

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.

After catching the second Mafia, Kyung Jun tries controlling the other survivors, seeking revenge on Jun Hee and those who voted for Woo Ram over him. Kyung Jun and friends grow suspicious of Yoon Seo's knowledge of the security cameras.

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.

Students find another body, intensifying fear as more die regardless of Mafia status. New clues implicate two possible targets before another vote, building tension.

Jin Ha and Kyung Jun are killed, increasing the students' fears. Doubting information, they feel urgency to expose the Mafia for self-preservation. So Mi suspiciously eyes Na Hee, foreshadowing an imminent, dramatic twist.

Na Hee's undercover identity exposed, endangering her life. Blaming herself for friends' deaths, she vows to take down the Mafia no matter what. Clues about the Mafia emerge but are inconclusive. The voting process makes Na Hee a target.

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.