Just outside Philadelphia, three girls — Claire, Marcia, and Casey — are waiting in the car of Claire’s father to be driven home from a birthday party. They are kidnapped by a man, who imprisons them in a secret underground lair beneath the Philadelphia Zoo. It becomes clear that this man has dissociative identity disorder (DID) rooted in his history of childhood abuse and abandonment. The man has been living with his 23 distinct identities (“alters”) for several years with help from his therapist, Dr. Karen Fletcher. One of these personas “Barry” has been in control of deciding which personalities get to control the man’s body; lately he has not allowed two others, “Dennis” and “Patricia” to have a turn due to the former’s tendency to harass young girls and the latter’s belief in a mysterious 24th yet-to-manifest alter called “The Beast” who plans to rid the world of the “impure”, those who have not suffered.
The girls meet “Patricia,” who protects them from “Dennis” to save them for “The Beast,” and a 9-year-old boy alter named “Hedwig,” whom “Patricia” and “Dennis” manipulate to shut out the other alters. The girls try to escape by going through the vents, manipulating the various alters, and communicating with another individual via a walkie-talkie, but all of their attempts fail.
During a session, Dr. Fletcher questions the man about an incident where two teenage girls had placed his hands on their breasts while they were drunk, and becomes concerned when she realizes that “Dennis” has been impersonating “Barry.” Dr. Fletcher goes to the lair to talk to “Dennis” when he claims to have made contact with “The Beast,” but she discovers Claire, so “Dennis” incapacitates and imprisons Dr. Fletcher. “Dennis” goes to the train station and boards an empty train car, where “The Beast” takes over. “The Beast,” who displays enhanced strength and animalistic tendencies, returns to his lair, thanks Dr. Fletcher for her help, and crushes her to death.
“The Beast” savagely feeds upon and kills Claire and Marcia before approaching Casey, but with posthumous instructions from Dr. Fletcher she calls out the man’s full name, (Kevin Wendel Crumb) which brings forth his original personality. Upon learning of the situation and realizing that he has not been in control of his own body for two years, the horrified Kevin begs Casey to kill him with a shotgun he has hidden. This prompts all 24 personalities to fight for control, with “Patricia” the victor. She tells Casey that “Kevin” has been made to sleep far away and will not awaken now, even if his name is called. While “Patricia” returns control to “The Beast,” Casey retrieves the shotgun and a box of cartridges and escapes into a tunnel. She manages to shoot “The Beast” twice before running out of ammunition, but he only sustains minor wounds. He moves closer to murder her, but stops when he sees scars across her abdomen and chest, which are related to her abuse from her uncle and legal guardian, John, both before and after her father’s death. Because she is “pure,” “The Beast” spares Casey’s life, and she is rescued the following day by the police.
In another hideout, “Dennis,” “Patricia,” and “Hedwig” discuss the power of “The Beast” and their plans to change the world. At the Silk City Diner, several patrons watch a news report on “The Beast’s” crimes, with the correspondent mentioning that his numerous alters have earned him the nickname “The Horde.” A waitress notes the similarity to a criminal in a wheelchair incarcerated 15 years earlier, who was also given a nickname. As she tries to remember it, David Dunn, sitting next to her, replies that it was “Mr. Glass.”
