Stephanie Aggiston, a teen from the countryside, is sent to Starkwater Hall Boarding School for the summer to brush up on advanced French at a prestigious private girls’ academy. She finds the school’s refined atmosphere a touch snobbish, but she manages to carve out friendships with a few classmates, including Marita Armstrong, Calli, and Shama, the latter of whom is introduced as a Saudi princess and her roommate. The campus, with its polished routines and hidden tensions, becomes the backdrop for a growing unease as the days drift by.
Soon after, a string of murders unsettles the tranquil routines of Starkwater Hall. Enter Det. Russ Kemper, who arrives to investigate and brings a clear-eyed suspicion to the proceedings. As more girls are killed, fear ripples through the dormitories and open-air courtyards, and the search for the killer intensifies. Stephanie decides to take matters into her own hands, determined to unmask the culprit before more lives are claimed. She enlists the help of a newly arrived, striking stable boy named Eddie Fox, who is drawn to her because she doesn’t fit the mold of the other girls. As the pair grow closer, suspicion shifts toward Eddie, who becomes a prime suspect in the eyes of the investigators and the school alike.
With the summer drawing on, the threat of danger weighs heavier, and the school begins the slow process of evacuating the students. Marita Armstrong is seized on the very day she is set to depart, kidnapped by the campus janitor, Robert Hartigan. Hartigan believes, bizarrely, that his daughter is Marita’s mother—a delusion that twists his actions on campus. When Marita manages to signal for help from his quarters, Stephanie signals Kemper to follow her as she heads out “alone,” drawing Hartigan’s attention and allowing Kemper to move in for an arrest and to rescue Marita from a perilous trap.
The tension peaks when Stephanie is ambushed and chased by a shadowy figure dressed in black. The pursuit ends with a shocking reveal: the assailant is Kemper, the supposed detective on the case, who turns out to be the actual killer. The first death, that of Tember Logan, is ruled an accident—a drowning that seeds the killer’s idea to continue with other murders as a twisted form of revenge against a figure named Miss Wade. The revelations unfold to expose Miss Wade as the killer’s mother, the person who abandoned him years earlier, fueling his murderous campaign in an effort to ruin the reputations of both the school and her. In the end, Kemper is confronted with the truth and taken into custody, his murderous arc brought to a close as the campus breathes a sigh of relief and the surviving students return to the quieter rhythms of school life.
