The story revolves around Reverend Cotton Marcus, a character depicted by Patrick Fabian, who resides in Baton Rouge, Louisiana, alongside his wife and son. Once a firm believer in his ministry, Cotton's faith begins to waver after a tragic incident involving the death of an autistic child during an exorcism — a situation that resonates painfully with him, reminding him of his own disabled son. His revelation leads him to acknowledge that the healing of his son stems from science rather than divine intervention.
In a bid to expose the practice of exorcism as a hoax, he agrees to participate in a documentary project, collaborating with a film crew that includes producer and director Iris Reisen and cameraman Daniel Moskowitz. Randomly selecting a letter from a person requesting an exorcism, his quest brings him to the troubled family of Louis Sweetzer who believes his daughter Nell, played by Ashley Bell, is possessed. Upon arrival, Marcus encounters hostility from Nell's brother, Caleb, who wishes for them to leave immediately. Adding to the complexity, Marcus uncovers that the livestock on the farm have also been mysteriously harmed.
In hopes of tricking the Sweetzers into believing in his prowess as an exorcist, Cotton sets up hidden speakers and electronic devices before performing the ritual. Post-ritual, he and his crew are convinced that they have treated Nell's condition, which they assume was misinterpreted as demonic possession. However, their certainty is shaken when Nell later appears in Cotton's hotel room, prompting him to seek medical clarification for her state. Despite seeking the expertise of a doctor who finds Nell physically sound, her father’s reaction to her supposed mental instability raises further questions.
As tensions within the Sweetzer household escalate, with Nell being chained for her violent behavior, Marcus and his crew delve deeper into the mystery, resulting in eerie and unsettling discoveries. They encounter disturbing artwork created by Nell that foreshadows potential doom for themselves. The pressure mounts when they learn Nell is purportedly pregnant. This revelation sparks a heated argument between Cotton and Louis regarding therapeutic versus supernatural solutions for Nell.
After a violent outburst from Nell, Cotton resolves to conduct another exorcism to protect her from her father's frenzied attempts to control her. They relocate to the barn, where the situation escalates dramatically, and the demon's influence over Nell is palpably sinister. During a climactic confrontation, Nell reveals the truth behind her violent episodes, exposing a tangled web of shame and deceit.
Determined to solve the riddle of Nell's behavior, Cotton and his crew return to the farm only to find harrowing signs of a dark ritual. Surrounded by occultists led by Pastor Manley, they witness a terrifying ritual where Nell is subjected to unspeakable horrors. As the tension reaches a fever pitch, Cotton's faith sparks a desperate attempt to confront the evil surrounding them, risking everything in a climactic showdown as devastating truths unfold, and the fateful night plunges into chaos and terror.
