Kevin Wendell Crumb, a man struggling with dissociative identity disorder (DID) rooted in his history of childhood abuse and abandonment, has been managing living with his 23 distinct personalities well for several years with help from his therapist, Dr. Karen Fletcher. The most dominant of the desirable alters, "Barry," has been in control of deciding which personalities get to control Kevin's body, and lately he has not allowed "Dennis" or "Patricia" to have a turn due to the former's tendency to harass young girls and the latter's belief in a mysterious entity called "The Beast" who plans to rid the world of the "impure," that is, those who have not suffered.
Dr. Fletcher is concerned when she realizes that "Dennis" has been impersonating "Barry" during their sessions after questioning him about an incident wherein Dennis had been left disturbed when two teenage girls had placed his hands on their breasts while drunk.