Lin Shuwei is a meek, straight-A student with no friends who becomes the target of relentless harassment from his classmates. Recently framed for stealing class funds, he endures public humiliation at the hands of a bullying group led by Ren-Hao Duan [Kent Tsai Fan-Hsi], his girlfriend Sihua, and members Liao Kuoheng and Yeh Weichu. When Lin confronts the gang at their hideout in the maintenance room of the school’s abandoned swimming pool, Ren-Hao mockingly admits to the theft, claiming he needed the money to tune Guo Feng’s motorcycle scooter. The moment turns darker as Sihua pushes Lin’s hand onto her breast, a calculated move to frame him as a pervert, and the group snaps a photo to spread the lie.
The confrontation ends with Lin narrowly escaping physical harm, but the gang intensifies their manipulation by restraining him with an animal-control pole and threatening him with a wall-dart. Despite the danger, Lin records the exchange on his phone. Yet their teacher, Ms. Lee, warns that accusing the bullies would only invite more trouble and implores Lin to take the blame. She then proposes an odd remedy: the four boys, now made to seem like the class’s most popular students, should team up and perform community service by helping the elderly.
What begins as a twisted charity project soon reveals a more sinister layer. The bullies and Lin take their service to an elder care home, but their visit devolves into abuse as the group paints faces and stages crude “chicken fights” between the residents. Lin is troubled at first, but the sense of belonging he gains from the gang gradually swallows his apprehension.
During their rounds, they encounter a locked box in the care home and discover a pair of flesh-eating ghouls living in the elevator shaft. The younger ghoul is later identified as Lee Shiu Jen, who went missing in 1985, and she possesses remarkable, unnatural abilities—regeneration and the power to climb walls. The boys capture the younger ghoul and subdue her with a portrait of Sun Yat-sen to keep her immobilized, while the older sister, the Adult Monster [Eugenie Liu], mourns the disappearance of her sibling.
The boys’ curiosity about the monster grows reckless. They collect blood samples and study the ghoul in the school lab, noticing their cells lack red blood cells and carry eggs or spores. They also see that sunlight causes the creature’s cells to ignite, a weakness they exploit to keep the younger ghoul contained. Lin, digging deeper into the ghoul myth, learns from news articles that the two sisters may be tied to dark arts and old legends; Lin shares his findings with Ren-Hao, who dismisses the possibility, insisting the creature is a monster now and the purpose of keeping it is to feed their curiosity.
As discipline collapses, a brutal incident occurs during a class prank when Ren-Hao mercilessly beats a classmate. Ms. Lee attempts to intervene with spiritual guidance, but Ren-Hao’s anger erupts, and the bullies poison Ms. Lee’s drink with the ghoul’s blood. Later, Ms. Lee collapses during a basketball game, bleeding black from within and finally succumbing to a sun-triggered death. The incident is captured on video and aired on the evening news, which the elder ghoul sees, fueling a growing hatred and a thirst for revenge.
A drinking party to celebrate the teacher’s death follows. Sihua drinks Lin’s beverage, while Ren-Hao taunts Lin, warning he could kill him in a million ways. Lin, shaken and angry, lashes out at a convenience store clerk and steals from him, signaling a deeper retreat into desperation.
The elder ghoul begins targeting students in the school, cutting down those in uniforms and even slaughtering a bus filled with children. Sihua is on the bus when the elder ghoul strikes, and she manages to photograph the monster’s bracelet containing her sister’s teeth—a sign that seals her doom as the monster violently kills her. Grief-stricken, Ren-Hao rips out hundreds of the younger ghoul’s teeth, further fueling the escalating violence.
With the younger ghoul now a captive, the older sister intensifies her hunt, attacking after-school programs and slaughtering numerous students, sparing only a few siblings who escape. The remaining bullies decide to trap the monsters in the maintenance room, pouring gasoline into the space and planning to set a blaze. Lin, rebellious and increasingly disillusioned, secretly sabotages the plan by enabling an escape for the ghouls, telling the younger sister that he wants at least one thing done right in his life.
Guided by a trail of arrows, the younger ghoul follows the veteran’s room to the pool. Lin, confronting Wei Zhu Yeh [Tao Bo-Meng]—the group’s brutal ringleader—asks why they framed him in the first place. Wei Zhu’s chilling answer is simple: “it was fun.” Realizing the danger of becoming like his tormentors, Lin chooses to let the monsters go free instead of sealing their fate, even as the plan to trap them collapses.
When the elder ghoul closes in on the maintenance room, the younger ghoul is revealed to be unable to escape the sun’s power. Lin opens a window to sunlight, and the younger ghoul bursts into flames, burning from the sun’s touch. The elder sister—desperately shielding her sibling—tears at her chains, sacrificing herself and her fingers to protect the younger one. The two sisters burn to death in each other’s arms, a poignant, tragic end to a saga rooted in fear, cruelty, and a warped sense of belonging.
In the aftermath, Lin is relegated to lunch duty, tending the soup container for the class. Despite the bullies’ demise, the class continues to torment him. Lin reveals he has kept the vial used to poison Ms. Lee and has dumped the rest into the soup, contaminating the entire class, including himself. The final image is stark: Lin rises, screams as he combusts, and the room fills with the chorus of his classmates burning around him.
