A girl sleeps in a locked room. A viewing slot opens and a young man watches her. Loud music begins to play, waking her up.
In a lecture at Oxford University, Dr. Joseph Coupland (Joseph Harris) explains that he believes that paranormal occurrences are the outward manifestation of the chaotic things a person thinks. He shows footage from an earlier case on which he worked. David Q (Aldo Maland) was a normal boy until he started drawing pictures of Mr. Gregor, whom he believed made things happen. According to Joseph, Mr. Gregor was not real and was created by David Q because he could not handle his mental illness. He shows a picture of the girl in the room, who will be the subject of his latest experiment. When one of the students challenges how Joseph treats the mentally ill, the others shout him out of the lecture hall. Outside, Joseph talks to Brian (Sam Claflin), who is not an Oxford student but who has been hired to film the experiment. Joseph notices Brian's cross and asks if he believes in God. Brian explains the cross belongs to his mum; hes more confused when it comes to religion. The doctor welcomes him aboard.
In the flat, Joseph has rented to conduct the experiment, Brian films Chrissy, an Oxford student who met Joseph when she volunteered to be his test subject. Chrissy (Erin Richards) says she has a lot of issues, but Joseph found someone more tortured: Jane Harper (Olivia Cooke). Chrissy shows Brian footage of Jane, the girl in the locked room. Off camera, Joseph asks Jane questions about Evie, a doll (as well as the manifestation of her paranormal feelings). Jane wants to sleep, and something leaps out from the corner of the room, frightening the person holding the camera. Chrissy laughs when Brian jumps and sends him upstairs to meet Jane. Harry, another student assistant, shows Brian a chart documenting how often Jane sleeps. According to Harry (Rory Fleck-Byrne), Jane is suicidal and has no memory of her past. She bounced around from foster home to foster home until Joseph met her. When Brian looks through the slot in the door, he sees Jane is awake, and she invites him inside. She has promised the doctor she will not try to kill herself again hopes she can be cured. She seems to like Brian and asks him to tell Joseph she is ready to sleep again. When Brian talks to Joseph, Joseph cautions him never to corrupt the experiment. He believes if he can isolate Evie and destroy her, he can cure Jane of her mental illness.
The next day, Joseph gives Jane a doll. Jane says she is not Evie because she looks dead inside. Once her foster parents told her she was dead inside. She wants to sleep forever. Before David Q, the doctor used to disprove the existence of ghosts. He reiterates that David Q created Mr. Gregor through the telekinetic chaos in his mind, the same way Jane created Evie. Footage of David Q shows the young boy banging his head against a wall and sobbing because he cannot summon Mr. Gregor. David Q's mother ended their sessions after David began harming himself, and later they discovered that Mr. Gregor was a monster from a children's tale David read. Now David is living a normal life, proving Joseph's theory.
Oxford pulls Joseph's funding for the experiment. A neighbor complains about the noised caused by the loud music, and the doctor must relocate the experiment to a large house in the countryside. He remains optimistic about the experiments prospects. After they move in, Harry shows Brian some of the equipment they will use to track a rooms telekinetic activity. An EF scanner will be able to monitor the energy levels in a room and will show them when Evie comes and goes. Chrissy and Harry build Jane's cell and move her inside. She stares out a window at the backyard. Later, Brian brings her a flower to brighten her room, and she smiles briefly. Jane tells him she has been a disappointment and needs to find Evie. Later, the doctor injects her with a drug that sends her into a dreamlike impressionable state. He hooks Jane up to another EF scanner that reads her energy levels and pushes her to find Evie. Jane murmurs that she only feels lost and alone. The doctor becomes frustrates with her and threatens to send her back to the a force knocks the camera from Brian's hand, interrupting Joseph's rant. The EF scanner shows Evie manifested in the room. Later, Joseph and the students and Brian celebrate with champagne.
That night, Harry studies the heightened EF scan from earlier outside of Jane's room. Chrissy, with whom he is sleeping, eventually drags him off to bed. Harry tells Brian to keep Jane awake. Brian turns up music to blot out the sound of Chrissy and Harry having sex; then something crashes in their room and they scream. Brian rushes to investigate, and he finds out the bed broke under them. Brian returns to the station outside of Jane's room, and he decides to check on her. She is standing right at the viewing slot. Jane walks further into her room and takes off her clothes. Brian backs away and finds Joseph watching him. The doctor looks into Jane's room through, telling Brian he will take over.
The next day, Brian films Jane through the viewing slot in her door. Her room is dark, she is hunched over her doll, and she is making odd jerking motions. Harry turns the lights on, and they see Jane is ripping the hair out of her doll. He hands are covered in blood. Chrissy rushes into the room and pulls a bobby pin from Jane's hand; she cut a symbol into her wrist. Chrissy asks Jane why she would wreck her doll. Jane lunges at Chrissy, and Brian pulls her off the other woman. Then Jane kisses Brian and tries to undo his pants. The doctor runs into Jane's room and expels Brian. When Joseph comes out, Brian apologizes. Joseph says Jane is aware of his attraction to her.
Another day, another experiment. Jane is injected again, and the doctor pushes her to find Evie. Jane mumbles about a fire, its heat, of everyone around her burning. Joseph tells her she is safe and believes Evie has joined them. He asks if she is afraid of the fire and slowly slides a candle under her arm. He continues to push Jane for answers about Evie, but when the flame begins to burn her arm, Brian yells at Joseph to stop. Jane is pulled from her state, Evie disappears, and the doctor is furious with Brian. That night, someone knocks at Brian's door. No one is outside in the hallway. He explores the house and eventually comes to Jane's room. He opens the viewing slot but cannot see her. Brian unlocks the door and finds Jane pressed against the wall right by it. She has cut herself again.
Morning comes. Brian sees Chrissy and Joseph walking in the backyard. Chrissy wants to take Jane to a doctor, but Joseph feels that would ruin the experiment. He thinks they are close to isolating Evie from Jane and destroying her, thereby curing Jane of her mental illness. The potential benefits the experiment could bring are too great for it to end now. Joseph flirts with Chrissy to distract her and then presses her against a tree, kissing her deeply. Brian looks up to Jane's bedroom window, where she is watching to couple too. Brian films Jane as she bathes. Jane says she and Evie could be friends, as they feel the same age. She also believes she was often naked as a child. She asks Brian if he finds her naked body attractive, but he turns the camera away. He tells Jane he could help her leave, but she doesn't want to disappoint Joseph.
The doctor instructs Chrissy to double Jane's dose of the drug when she injects her. Everyone sits in a circle around a table and holds hands. Joseph asks Jane about the fire, and she cries out that its so hot. She grabs her doll and screams that its burning, burning, burning. Smoke billows from her hands, and her flesh sizzles. The doll somehow burns her hands. Brian continues to film, and as Harry and Joseph try to calm Jane, she turns into someone bloody and on fire. Later, Brian tells the doctor he caught the change on film. Joseph is convinced she manifested Evie in the room, a so-called teleplasm. He wants to push forward, but the students and Brian vote to give Jane a break. They spend a sunny afternoon playing outside. When they return to the experiment, with Jane in her dreamlike state, she cannot find Evie. The doctor wants Evie to go into the doll, so they can talk to her, but Jane knows they're lying.
The experiment continues to deteriorate, and everyone at the house becomes more isolated from the outside world. One night Chrissy complains to Harry that she has stopped going to classes. The power in the house goes out, and when everyone goes to check on Jane, she has escaped from her room. The doctor, the students, and Brian hear someone stomping around in the attic. They find Jane crouched over a little girls bed. She peels back the bedding, revealing the scorched outline of a child's body. Jane whispers to the outline, presumably made by Evie, that she will protect her and take her away. When she reaches out for Evie, a force slams into her and cuts the same symbol into her stomach.
Joseph recognizes the symbol as a sigil, a mark used by devil worshipers. He believes the students or Brian have corrupted the experiment by telling Jane about Satanic rituals. Now she believes she is possessed by the devil, not Evie, her telekinetic manifestation. Chrissy believes Evie came from somewhere other than Jane's mind, and Harry suggests Jane might be a kind of supernatural conduit. From her room, Jane tells everyone she wants to continue the experiment. Harry and Chrissy decide to return to Oxford, because they have lost faith in Joseph.
Brian asks Jane if she has ever seen the sigil before. Jane replies that she never felt comfortable as herself before coming to the house. When people called out her name, it felt like it wasn't hers. She asks Brian not to lose faith in Joseph, in Evie existence, or in her. She kisses him. Jane thinks for the first time in her life there is a way for her to be happy, but Brian won't be with her until she is well. Eventually he decides he must go and do research at Oxford. He promises Jane he will return and lies to the doctor, saying he needs more equipment.
At the Oxford library, Brian sees the student who left the experiment. He tries to ignore the student, and the other man calls him a quiet one, a secretive acolyte of the doctor. Killing a girl is a wonderful way to create a ghost, he calls after Brian. Brian finds a book that links the sigil to a group of devil worshipers called the Lilitu. They operated out of the same house in which he, Jane, and the doctor now live. Elsewhere in town, Chrissy takes a bath. The water begins to bubble and boil. She stumbles from the bath, but the walls are searingly hot to the touch. Chrissy screams as the room fills with smoke. Harry hears her and breaks the door open.
Everyone converges back at the house. Chrissy believes Evie tried to kill her. She and Harry want to admit Jane to an asylum before Evie strikes again. Brian tells the doctor and the students about the Lilitu. The cult predates Christianity, and its 1960s reincarnation was run by David Dwyer. Davids young daughter Evie was born with a sigil birthmark, so David believed she was a reincarnation of their idol. They isolated Evie, much like the doctor has done to Jane, and in June 1960 she burned down the house. Everyone inside was killed. The doctor believes Jane learned about the cult from her parents and has named her telekinetic manifestation Evie because of that. The doctor banishes Brian from the experiment.
Brian borrows Chrissy's car so he can take Jane away from Joseph. When he goes to fetch Jane, he finds her watching more footage of David Q. It turns out David was Josephs son and that Joseph never cured him. Instead, David killed himself as a teenager. Jane screams angrily at the doctor for lying to her, but everyone is distracted from the revelation when sigils are burned into their skin. Chrissy tries to flee, but an invisible force grabs her and kills her and Harry. Jane runs away, and when Brian finds her, she is looking at her file. She figures out she was Evie Dwyer and that the trauma of the fire caused her to forget her past. Brian wants to save her, but Evie doesn't believe she deserves to be saved, because she murdered the Lilitu. Something possesses her, and she attacks Brian. Joseph jumps out and injects her with a drug. He also knocks Brian out.
When Brian regains consciousness, he is tied up and Joseph has strapped Jane to a table. He plans to kill Jane by injecting her with a high dose of Chrissy's drug, then revive her as Evie by pumping adrenaline into her heart. He kills Jane with the first injection but wont bring her back right away. Brian escapes his bonds and attacks the doctor. He manages to knock Joseph unconscious and injects Jane with the adrenaline. She comes back to life. He smiles at her and asks, "Jane?" She shakes her head and says she sorry. Jane then throws him from the room and locks it from the inside. As Brian begs her to let him in, she combusts, and the room becomes engulfed in flames. As Jane dies, something flies from her mouth at Brian, and the film stops.
Present Day, 2014.
Brian, now middle aged and strapped to a chair, answers a doctors questions. He keeps telling the doctor that Evie killed Chrissy, Harry, Joseph, and Jane, but the doctor doesn't believe him. He says Brian killed those people and reminds him that the film burned up in the fire he set. Smoke begins to billow from Brian's hands. It is clear that he has now become Evie's host.