Don Wallace (Finn Cole), a teenager hailing from a working-class background, reluctantly finds himself enrolled at the prestigious Slaughterhouse boarding school, compelled by his mother, Babs Wallace. Slaughterhouse, with its peculiar name, is an exclusive public school where Don grapples with feeling out of place among his affluent peers. His mother assures him that attending this school will afford him the chance to connect with future leaders, actors, and pop stars. The transition is particularly difficult for him, especially since he arrives mid-semester.
At Slaughterhouse, he meets his housemaster, Mr. Meredith Houseman, along with his new roommate, Willoughby Blake. An air of mystery surrounds Blake’s former roommate, Viscount Seymour, as both Houseman and Blake sidestep questions about him, leaving Don curious and concerned. The school features various houses, including the girls' house Andromeda, the nerdy Xenephobes, the athletic Olympus, and the underachieving Sparta/Farta.
The historic castle, once a noble palace surrounded by dense woods and a small lake, forbids swimming and exploring the forest. Among Don's classmates, Clegg, a snobbish prefect, and Caspar De Brunose, the head boy, dominate the social scene. Meanwhile, Houseman harbors feelings for Audrey (Margot Robbie), a former school nurse now engaged in humanitarian work in war-torn regions, who he communicates with regularly through video chats. His longing for Audrey highlights Houseman’s internal conflict over his past decisions.
Adding to the school's tension, a tragedy looms overhead; the suicide of Seymour last semester haunts the corridors. The principal, The Bat, announces a controversial fracking project on school grounds, promising financial gains from the shale gas beneath the estate, which will fund luxurious amenities for the students. However, this venture soon leads to the emergence of a perilous sinkhole filled with gas, drawing attention to the folklore of a knight who once vanquished a beast in exchange for the land on which the school stands.
As Don develops feelings for Clemsie Lawrence, he faces hostility from Clegg, who seems to resent Don's interest in her. With Blake's guidance, Don learns about Clemsie's status as the most desirable girl in school and his own lack of social leverage. A fateful breakfast run turns chaotic when Blake and Don take a shortcut through the woods, leading them to the ominous sinkhole. Nearby, they encounter Woody Chapman, who is spearheading protests against the fracking operations and warns them about the lurking horrors within the sinkhole.
A dramatic twist occurs when Don catches Clemsie in a compromising situation with another student, pushing him deeper into a spiral of conflict with Clegg. The atmosphere becomes increasingly foreboding with whispers of Seymour's tragic fate and the aggressive anticipation of a vengeful force from the sinkhole. As tensions heighten, Clemsie, Don, and Blake come face-to-face with bizarre rat-like creatures that lead to chaos, further endangering their lives when a massive earthquake rattles the school.
In a desperate attempt for survival, the group seeks refuge from the rampaging rats, encountering betrayal and deadly confrontations. Houseman takes charge, realizing they need to fight back. The story races toward a gripping climax as alliances are tested, loyalties are discovered, and ultimately, the school's longstanding traditions collide with an unexpected evil. Fleeing into the underground tunnels, they engage in a frantic battle for survival, with heartbreak and bravery flashing against the backdrop of chaos, culminating in an explosive finale that leaves the future of Slaughterhouse hanging in the balance.
As devastation unfolds, specters of the school's past loom large, and the students are forced to confront the ghosts that haunt their hallowed halls, ultimately leading to profound transformations in their lives and relationships. Don and Clemsie's bond strengthens amid the turmoil, and as they share a kiss, they realize they’ve fought not just for survival, but for their futures as well.
