Within an elite high school class of twenty students preparing for college, the dynamics crackle with tension, ambition, and danger. Kang Hyeon stands out as the rebellious figure, while his counterpart Kang Yi-na is the friend at the center of the group, and the class clown Kang Hyun masks his warmth with a jokey surface. A few days after troubling incidents involving a fellow student, Beom experiences a ghostly presence haunting his paper, and the class teacher Hwang Chang-wook notices unsettling scars on his hand. Then a TV broadcast jolts everyone: the top-ranking student, Min Hye-yeong, is shown trapped in a glass container that is slowly filling with water, and a stern voice over the PA announces they are now in a deadly death game. The rules are crystal clear—questions will be asked, answers must come in time, and leaving the school is strictly forbidden for them and their two teachers, Hwang Chang-wook and Choi So-yeong. The first question is asked, and the class fails, sealing Hye-yeong’s peril.
As the warning proves real, the group ventures to escape, only to discover the grim reality: a teacher and the hall monitor have been beaten to death, leaving the lone security guard to patrol the corridors. The students split up, scrambling to answer the questions while desperately hunting for an exit. With each passing puzzle, a brutal death follows, and the clues point backward to Kim Ji-won, a student who died earlier in the year under murky circumstances. The ominous math of the game reveals that the casualties mirror the ranking from the last exam, turning the room into a grim ledger of who stood where academically. Beom, who had earlier tangled with I-na and was later placed in a mental facility, makes a break for freedom and vents his rage, attempting to strike I-na, only to be shielded by Kang Hyun, who sacrifices himself in the process. After Beom is killed by Chang-wook, I-na learns that her friend Yoon Myung-hyo has also been kidnapped by the mastermind.
The survivors push on, solving the trap only to watch the killer snuff out Myong-hyo by dropping her from the roof. In a dramatic confrontation, Hwang Chang-wook climbs the roof and strangles the killer to death—the person revealed to be Ji-won’s mother. A memorial is held for the fallen, but the danger isn’t over: I-na is kidnapped yet again, and one final question is posed: who killed his daughter? The now-demented Chang-wook steps forward, only to be killed by the true mastermind, who turns out to be the security guard himself. Choi So-yeong then frees I-na, ending a terrible chapter.
Flashbacks peel back the layers of the tragedy. Ji-won, the student who strived to enter the elite class, faced a brutal truth: the parents of the killed students bribed Chang-wook for the entrance-test answers, giving their children an unfair edge. When Ji-won confronted Chang-wook, she offered to stay silent in exchange for help with her father’s failing business; he refused, and Ji-won was strangled. The family’s fortune collapsed afterward, loan sharks moved in, and Kim—Ji-won’s mother by the account of the cast—worked with her husband to orchestrate a vendetta against Chang-wook and the other students deemed responsible. The sole witness, Beom, lived with the belief that Ji-won’s presence haunted him from that day forward. As the two families behind the crime circle back, they use their daughters’ recordings to justify their broad revenge.
The film closes with a chilling implication: I-na emerges at the top of the class ranking, and Beom’s assaults feel hollowly spectral, as if Ji-won’s influence has lingered from the start. The brutal cycle hints that the extraordinary pressure of the elite class and the lengths families will go to secure their status have left a dark, indelible mark on everyone involved.
