Casey Cooke is an emotionally withdrawn teenager, having been molested as a child by her uncle John, who became her legal guardian after her father's death. While hanging out with her classmates, Claire and Marcia, the three are kidnapped by Kevin Wendell Crumb, a man suffering from dissociative identity disorder. Kevin has been evaluated by psychologist Dr. Karen Fletcher, who discovered that he formed multiple personalities as a means to cope with the abuse he suffered at the hands of his mother. While Kevin appears otherwise stable, Dr. Fletcher has identified 23 distinct personalities within Kevin; in his mind, these personalities sit in chairs in a room, waiting for their turn "in the light" (controlling Kevin's body) as directed by "Barry", the dominant personality. She has also found that Kevin's physiology changes when a new personality comes into the light. Recently, "Barry" has refused to allow "Dennis" or "Patricia" their turns, in part due to Dennis' tendencies towards bothering underage girls and Patricia's undesirable traits, and also because both appear to worship "The Beast", a yet-unseen 24th personality. Regardless of which personality is controlling Kevin, Dr. Fletcher can call forth Kevin's own personality by speaking his full name to him.

Kevin takes the girls to his underground quarters and secures them in a cell. They recognize his dissociative disorder, and Claire attempts to use this against Kevin to escape but is caught by "Dennis" and segregated from the others. Over several days, Kevin leaves them locked up while he goes off to work and attends appointments with Dr. Fletcher. During these meetings, Dr. Fletcher recognizes that "Dennis" has seemingly taken over the dominant personality from "Barry", though he tries to act as "Barry" to fool her. She recalls an incident that "Barry" told her about a month before, where two teenage girls had walked up to him, taken his hands and placed them on their breasts, which may have triggered "Dennis" to take over. Dr. Fletcher suspects that Kevin, as "Dennis", is responsible for the three missing teenagers.

During this time, Marcia also attempts to escape but is caught by "Patricia" and segregated. Casey befriends "Hedwig", a young boy personality of Kevin's; it is later revealed that "Hedwig" has complete control over "the light", and has provided "Dennis" and "Patricia" with control. She gets "Hedwig" to let her out of her cell to see his bedroom, believing that there may be a means of escape through "Hedwig's" window but finds that it is only a drawing of a window. She is able to convince "Hedwig" to give her a walkie-talkie. She uses it to call for help, but the person at the other end of the walkie doesn't believe her, thinking that someone is playing a joke. Before she can convince the person that she is serious, "Patricia" takes over Kevin's body and subdues Casey.

Dr. Fletcher shows up at Kevin's home, where he reveals that "The Beast" is real and that he has met him. Suspicious of "Dennis", Dr. Fletcher feigns needing the bathroom to look around his home, where she finds Claire in her cell, but before she can do anything, "Dennis" catches her, sprays her face with a sedative in an aerosol can, and secures her in a separate room. He then leaves. While out, "The Beast" personality takes over for the first time, giving Kevin super-human abilities, while Claire and Marcia attempt to escape from their cells. Dr. Fletcher regains consciousness long enough to write out Kevin's full name on a piece of paper before "The Beast" returns and kills her. Casey escapes from her cell, only to find that "The Beast" has already attacked and eaten Marcia and watches in horror as he does the same to Claire.

Casey finds Dr. Fletcher's body and her note on the piece of paper. "The Beast" approaches her, but she calls out Kevin's full name, bringing Kevin forth. Upon learning of the situation and realizing that he hasn't been in control of his body for two years, Kevin instructs Casey to kill him with his shotgun. Following this statement, all 24 personalities begin warring for control of "the light", but shortly thereafter "The Beast" personality takes hold again. Casey escapes from his quarters into an underground tunnel and shoots him twice, but the shells have little effect. She locks herself in a small caged area. "The Beast" finds her and is about to pull the bars open to attack her but sees faded scars across her body, a sign of her past emotional turmoil and self-harm. Having previously declared his plans to rid the world of the "untouched", those whose hearts are impure because they have never suffered in their lives, he considers Casey to be "pure" and spares her, disappearing from the tunnels.

Sometime later, Casey is rescued and learns that she was being held at the Philadelphia Zoo, where Kevin had been an employee. Casey is asked by a police officer if she is ready to return home with her uncle. She hesitates to answer. In another hideout, "Dennis", "Patricia", and "Hedwig" exert collective control over Kevin's body and admire the power of "The Beast" and their plans to change the world.

In a diner, several patrons watch the news as a reporter states that Kevin's personalities have earned him the nickname "The Horde". A female patron notes the similarity between Kevin and a wheelchair-using criminal who was incarcerated 15 years earlier. As she tries to remember the nickname he was given, the man sitting next to her tells her that it was "Mr. Glass".

https://en.wikipedia.org/wiki/Split_(2016_American_film)
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