A girl lies in a securely locked room. When a viewing slot opens, a young man peeks in just as loud music jolts her awake. May, 1974. During a lecture at Oxford University, Dr. Joseph Coupland (Jared Harris) presents his belief that paranormal events are simply manifestations of a person's chaotic thoughts. To illustrate his point, he shares footage from a previous case involving a boy named David Q (Sam Claflin), who once seemed normal until he started drawing Mr. Gregor, a figure he believed was influencing his reality. Joseph asserts that David Q's struggles were a result of his mental illness and that Mr. Gregor was merely a figment of his imagination. He then introduces the audience to Jane Harper (Olivia Cooke), the subject of his current experiment, someone even more troubled than David.
When a student questions Joseph’s methods regarding the mentally ill, the class descends into chaos. Afterward, outside the lecture hall, Joseph strikes up a conversation with Brian, who, despite not being an Oxford student, has been recruited to document the experiment. Joseph queries him about his cross, which belongs to his mother, leading to a discussion on faith and belief.
The setting shifts to the flat where the experiment is taking place, revealing that Joseph has enlisted Chrissy (Erin Richards), an Oxford student, as a test subject. Chrissy shares her own issues but is quick to point out Jane, who is in the locked room. Joseph probes Jane about a doll named Evie, which symbolizes her psychic turmoil. Scary occurrences unfold as Jane hopes for release from her pain but struggles with her traumatic past.
A dark atmosphere surrounds the experiment, prompting Joseph to push boundaries in pursuit of truth. He engages in his manipulative techniques while pushing Jane into emotional turmoil, revealing footage of David Q's deteriorating mental health, similar to Jane's. As Joseph loses funding from Oxford, he relocates the experiment, maintaining hope against odds.
Once in the countryside, tensions rise. Jane expresses a desire for connection while trapped in her solitude, and Joseph’s obsession with isolating Evie intensifies. Dark events ensue, including self-harm, as Joseph seeks to prove his theories at all costs.
The psychological experiment begins to unravel. Jane's condition worsens, and the group experiences unsettling experiences, like Jane's bizarre actions and grim harbingers of death. Despite Brian's concern for Jane, Joseph's chilling demeanor resists compassion and clarity, leading them deeper into danger.
As the experiments escalate, the group learns about dangerous rituals linked to a previous tragic event involving the Lilitu sect. While trying to control Jane’s manifestations, Joseph inadvertently fuels the chaos, and Jane starts to recall her past, realizing she was once Evie Dwyer, born into horror.
The final confrontation leads to violence and despair. As Brian strives to save Jane, chaos reigns, and tragic resolutions unfold. The experiment spirals out of control, resulting in deaths and the fiery expression of Jane's inner turmoil.
Fast forward to 2014, with Brian now an older man trapped in a chair, recounting his tale to a skeptical doctor. Despite Brian's insistence that Evie is responsible for the deaths of Chrissy, Harry, and Joseph, the doctor is convinced of Brian's guilt. As the horror lingers in the air, smoke begins to emerge from Brian's hands, hinting that he has become a vessel for Evie's malevolence.
