The narrative unfolds through a series of escalating deaths after the whistle is sounded. Star athlete Mason "Horse" Raymore is incinerated from within by his charred Aztec doppelgänger in the opening. Months later, Mr. Craven confiscates and tests the whistle, leading his doppelgänger to accelerate his smoking-induced lung damage and fatally crush his lungs. At a gathering, Grace blows the whistle, cursing herself, Dean, Rel, Chrys, and Ellie. Grace is subsequently drained into extreme old age at a harvest haunt maze, Dean is crushed by severe car-crash impact trauma in his room, and Rel is shredded by his doppelgänger after hesitating to sacrifice drug-dealing youth pastor Noah.
To defeat the curse, Chrys and Ellie devise a clinical loophole: Ellie induces temporary cardiac arrest in Chrys using insulin, fulfilling her destined death vision (a syringe in her arm) without a fatal overdose. Ellie successfully resuscitates Chrys with adrenaline and glucagon. When an escaped Noah shoots at Ellie, he accidentally touches her spilled blood while her apparition is in the room, triggering the blood-transfer rule and causing his own future apparition to destroy him.
In a mid-credits twist, Chrys and Ellie attend a spring assembly believing they are safe. However, a freshman named Asha mistakes the recovered whistle for a pitch pipe and blows into the microphone, broadcasting the curse's scream across the entire packed auditorium.