The film stars James McAvoy, Anya Taylor-Joy and Betty Buckley and follows a man with 23 different personalities who kidnaps three girls.
Three teenagers, Claire, Marcia and outsider Casey, are kidnapped and held captive by "Dennis",
one of 23 personalities present in the mind of Kevin Wendell Crumb, a victim of childhood abuse
diagnosed with dissociative identity disorder.
Kevin's psychiatrist, Dr. Karen Fletcher, who cites studies in which alternate personality states exhibit distinct physiological and psychological profiles,
is concerned to find an e-mail from Kevin's protective personality, "Barry", asking for a meeting.
Over the years, Kevin has been treated, he appears to be stable: all of his personalities sit in chairs in a room, waiting for their turn "in the light",
controlling the body, while "Barry" controls who gets to go in the light.
Two personalities, "Dennis" and "Patricia", are kept out of the light.
In Dennis's case, this is because he likes to watch young girls dance naked, and he has both violent tendencies and obsessive–compulsive disorder.
Dr. Fletcher, through her studies, has reached a conclusion that people like Kevin can overcome their disabilities by moving to different personalities.
Later, in an attempt to explain his new personality, Kevin reminds her about a blind girl with split dissociative identity disorder who could see when she went into another personality.
She attends a medical conference online and tries to make her point; this could be a solution to all the disabilities.
Claire and Marcia conspire to attack "Dennis", but are dissuaded by Casey who appears to have experience with abuse and wants to wait for a better chance.
The girls realize Kevin's nature when "Patricia", who dresses as a woman when in the light, comes and assures them that "Dennis" is forbidden from touching them as they serve a greater purpose.
Casey seeks to befriend "Hedwig", a personality that claims to be a nine-year-old boy, who confides that they will be sacrificed to "The Beast", a rumored 24th personality,
and inadvertently reveals that the cellar has recently been reinforced to hold them.
Casey and Marcia barricade the door while Claire attempts to crawl through the fresh drywall, but "Dennis" emerges and stops her, locking Claire in a separate cell.
"Dennis" masquerades as "Barry" and attends a meeting with Dr. Fletcher, dismissing the e-mails,
but Dr. Fletcher notices discrepancies in his behavior and quickly realizes "Dennis" and "Patricia" have supplanted "Barry" as the dominant personality,
and they discuss the possibility of "The Beast" being real. Casey and Marcia later receive food from "Patricia", who brings them to the kitchen.
Seeing an opportunity to escape, Marcia hits "Patricia" with a chair and flees, but is caught and placed in a different cell.
Casey then continues to befriend "Hedwig", having earlier heard him mention a window in his bedroom.
"Dennis" once again visits Dr. Fletcher, and they talk about Kevin's father, who abandoned Kevin as a child aboard a train,
leading the other personalities to believe that "The Beast" is hidden inside a train car.
The personalities began manifesting to help Kevin cope with the abuse he was subjected to by his mother,
who suffered from obsessive–compulsive disorder. Arriving back at Casey's cell, "Hedwig" is persuaded to take Casey to his bedroom,
but Casey is distraught to learn it was only a drawing of a window.
She grabs a functional walkie-talkie nearby and radios for help from an unknown voice.
The unknown voice thinks it's a gag or she stole a workmate's walkie-talkie and dismisses her.
Casey is subdued by "Patricia".
Casey then experiences flashbacks of being molested by her uncle John,
who became her legal guardian after her father's death. She recalls an incident where she was molested while on a hunting trip with her father and uncle,
where she later contemplated killing her uncle with his shotgun, but hesitated and the opportunity was lost.
Dr. Fletcher becomes alarmed by a large number of e-mail messages for help that she receives from "Barry".
She rushes to Kevin's home and he opens up about a vague plan.
She suspects he is responsible for the kidnappings. She excuses herself to go to the bathroom and discovers Claire.
Before she can help her, she is caught, drugged and locked up by "Dennis", who then goes into a train car and becomes "The Beast", manifesting superhuman speed, strength, resilience and agility
Knowing that the only way to call to "the light" the real Kevin is to speak his full name, Dr. Fletcher writes it on a piece of paper before being killed by "The Beast".
Meanwhile, the girls separately try to escape from their cells.
"The Beast" first kills and cannibalizes Marcia, and then Claire,
while Casey stumbles upon Dr. Fletcher's corpse as well as her instructional note, and briefly calls Kevin to "the light" by speaking his name
. Horrified by his actions, Kevin directs Casey to where his shotgun is and orders her to kill him before his other personalities begin to take over,
switching from one to the other until "The Beast" returns and corners Casey after she gets the shotgun.
She shoots him at point blank range, to which he seems to be impervious.
"The Beast" then voices his plans to rid the world of the "untouched", those whose hearts are impure because they have never suffered in their lives.
"The Beast" prepares to devour Casey when he notices numerous scars on her torso,
and rejoices in the fact that she is "pure". Concluding that troubled people are exceptional, "The Beast" spares Casey and leaves.
Later, Casey is rescued by one of Kevin's co-workers and learns she was being held underneath the Philadelphia Zoo,
where Kevin works and lives, in the basement of the maintenance building.
The co-worker helps her pass a security shack and tells the officer about Casey and the other girls.
Casey is asked by a police officer if she is ready to return home with her uncle.
She hesitates, but her response is not shown
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, patrons listen to the media coverage of Kevin's crimes, for which he's been nicknamed "The Horde".
One of the patrons notes the similarity between Kevin and a terrorist who uses a wheelchair and was arrested 15 years prior.
The man sitting next to her is revealed to be an older David Dunn, who reminds the patron that it was "Mr. Glass".