The narrative unfolds across three intertwining timelines: "Rose," "Joan," and "Kat," which converge dramatically at the film's climax.
Rose
In February at the prestigious Bramford Academy, a Catholic boarding school located in upstate New York, students eagerly anticipate being picked up by their parents for an upcoming week-long break. Among them are Kat, a freshman, and Rose, a senior. Rose, suspecting she might be pregnant, chooses to deceive her parents about the vacation dates to buy some time. Meanwhile, Kat experiences a haunting premonition of her parents’ tragic demise in a car accident. That same night, Rose leaves to confide in her boyfriend about her pregnancy. Hours later, Kat receives a distressing phone call. When Rose returns, she discovers Kat in the boiler room, engaging in unsettling behavior and making cryptic statements about her parents' deaths, which leaves Rose shaken. After Rose exits, Kat's body begins to contort and convulse ominously in her bed.
The following morning, both Rose and a visibly ill Kat join the nuns for breakfast. Kat’s erratic behavior escalates as she suddenly vomits and hurls curses at the nuns. After a troubling phone call, the nuns instruct Rose to shovel the driveway "down to the earth" in preparation for the unexpected return of Headmaster Gordon. Upon his arrival, accompanied by a police officer, they stumble upon a chilling bloodstain on the wall, eliciting shock and concern from the men.
Joan
In a different timeline, a young woman named Joan arrives at a bus stop after having escaped from a mental institution. An amiable older couple, Bill and Linda, offer her a ride. They check into a motel, where Joan showers, revealing a scar on her shoulder—a remnant of a violent past where she was shot by a policeman. Bill admits that Joan reminds him of his late daughter, whose name is revealed to be Rose. The next day marks the ninth anniversary of Rose's death, a day her parents consistently pay tribute to the school. In a moment of recognition, Joan retreats to the bathroom, attempting to stifle her laughter. It soon becomes apparent that "Joan" is an assumed identity; the real Joan was a woman she murdered to take over her life.
Kat
As winter break approaches, Kat finds herself drawn into a sinister connection with an entity that appears alternately as a horned shadow and communicates with her in a gravelly voice. This malevolent presence warns her of her parents’ imminent deaths and urges her to commit murder. After a disturbing breakfast, the nuns receive the horrific news of Kat's parents' death. As Rose is sent outside to shovel the driveway, Kat tragically murders both nuns. Following the confirmation of her menstruation—indicating she is not pregnant—Kat gruesomely attacks Rose, killing and beheading her. Gordon and the policeman discover Kat prostrated in front of the boiler, surrounded by the severed heads of her victims. With a chilling declaration of "Hail Satan!" escaping her lips, she is shot by the policeman in her shoulder, revealing that "Joan" is none other than Kat. Subsequently, she is confined to a mental institution, where a priest attempts an exorcism. While the exorcism seems initially successful, Kat pleads with the demon not to depart, marking the beginning of her complex struggle with the darkness within.
Nine years later, the "Joan" now embodying Kat commits a grisly act of violence against Bill and Linda when they stop in Bramford. After beheading their bodies, she attempts to summon the demon she once lost in the boiler room of the academy. However, she finds the boiler cold and unattended, echoing her profound isolation. Abandoned and devoid of purpose, she leaves the academy, breaking down into tears in the middle of the road, accentuating her despair and the haunting legacy of her past.
