Casey (Anya Taylor-Joy) is a strange girl who seems to be out of tune with her peers. One day, she and her classmates Claire (Haley Lu Richardson) and Marcia (Jessica Sula) were preparing to take a car home. They were attacked and kidnapped by a mysterious man. The three were imprisoned in a dark and cold basement, facing an unknown and dangerous fate. The kidnapper of the three girls was a man named Kevin (James McAvoy). In fact, he is a multiple personality patient, and there are as many as 23 personalities that dominate his body. Fletcher (Betty Buckley) is Kevin's attending physician. She found that there were differences between the personalities in Kevin's body. What's worse, the 24th personality is slowly forming. This personality is strong, evil and cruel. Once he gains control of Kevin's body, not only will the three girls suffer, but Kevin himself will also be in danger of death.