Three teenage girls, Claire, Marcia, and Casey, are abducted by a stranger with an unusual affliction—23 distinct personalities reside within his mind. The girls wake up in an unknown, sealed-off room, the lair of Kevin, whose complex condition is being studied by his psychiatrist, Dr. Fletcher. As the girls attempt to escape, they meet several of Kevin's alters, including the stern Dennis, the fastidious Patricia, and the nine-year-old Hedwig. Each of these personalities has a different interaction with the captives, ranging from childish curiosity to methodical control.

As time progresses, the girls try to manipulate the more sympathetic personalities to aid their escape, while simultaneously learning about the nature of Kevin's condition. Dr. Fletcher grows increasingly concerned over Kevin's behavior, suspecting that a new, dominant personality, referred to as "The Beast," is emerging. The girls' survival hinges on their ability to outsmart the various personalities that control Kevin's thoughts and actions, all while piecing together clues and attempting to reach the outside world. Their struggle reveals not only the complexities of their captor but also unearths traumas and secrets harbored within themselves.