The Reverend Cotton Marcus (Patrick Fabian) lives in Baton Rouge, Louisiana, with his wife (Shanna Forestall) and son. Marcus is accustomed to performing exorcisms on “possessed” individuals, but his faith wanes as he reads of an autistic child being killed during an exorcism, reminding him of his own disabled son. He comes to realize that he attributes his son’s healing to science, and not to Jesus Christ. He agrees to take part in a documentary designed to expose exorcism as a fraud, working with a film crew consisting of producer/director Iris Reisen (Iris Bahr) and cameraman Daniel Moskowitz (Adam Grimes). At random, he chooses an exorcism request sent by farmer Louis Sweetzer (Louis Herthum), who claims his daughter Nell (Ashley Bell) is possessed by a powerful demon named Abalam. Prior to the exorcism, Marcus plants hidden speakers and electronic props, so he can bamboozle the family into believing he is driving out a demon. After the ritual, Marcus and his film crew leave, believing they have cured her of a mental state that was misdiagnosed as a possession.
That night, Nell mysteriously appears in Marcus’ hotel room. The team tries to contact Louis to get permission to film her, to no avail. In an effort to prove his point, Marcus takes Nell to the hospital for testing, hoping for a medical or psychological diagnosis proving demonic possession is not the cause of her illness. The doctor concludes that Nell is in perfect physical condition. Marcus goes to see Louis’ former pastor, Pastor Manley, and asks him questions regarding Nell and Louis. Pastor Manley informs Marcus that he has not had contact with the Sweetzers for a long time. In the morning, her father takes her home and chains her in her room for slicing her brother, Caleb (Caleb Landry Jones), in the face. While Louis takes Caleb to the hospital, Marcus and the camera crew further investigate Nell and the home. They find Nell chained to her bed and release her. That night there is a disturbance in the house, and Marcus and the crew hear the cries of a baby. They find Nell standing in a hallway. When they try to confront her, she goes into a bathroom, where she is found submerging a baby doll in water. After she comes out of her trance, the crew finds a drawing of a dead and bloodied cat.
That night, while Marcus and the crew are asleep, Nell steals their camera and goes into her room, placing the camera on a dresser as she pulls and distorts her face. Nell goes into her father’s barn where she corners a cat, beating it to death with the camera. She returns to the house and raises the camera over Marcus’s head, ostensibly to beat him to death with it. The rest of the crew stops her, unaware of what has transpired. They discover two more paintings of Nell’s. The first depicts someone who appears to be Cotton standing before a large flame, holding up a crucifix. The second shows what appears to be the dead bodies of all three visitors to the Sweetzer farm: Cotton being consumed by the flame he battled in the other picture, Iris hacked to pieces with an axe, and Dan decapitated. Louis comes home and hears an answering machine message from the hospital stating that Nell is pregnant. Marcus thinks Louis might have committed the act of incest, which he denies, insisting that Nell is a virgin and has been defiled by the demon possessing her. Tempers flare as Marcus insists that Nell needs psychotherapy instead of another exorcism. Louis orders the crew to vacate his property. While contemplating whether they should take Nell, they hear noises upstairs. They discover that Nell has climbed atop her dresser. As they try to calm her, she slashes Marcus’s hand with a knife and runs outside. The crew decides to leave; as they enter their van, they see Nell sitting on the porch. As Marcus approaches Nell, she tackles him and attempts to harm him. The struggle leads inside as Louis chases them with a shotgun. In order to keep Louis from killing Nell, Marcus agrees to attempt another exorcism.
During the exorcism, Nell’s body contorts and bends in ways that don’t appear to be humanly possible. Marcus confronts the entity that has possessed Nell, which introduces itself as Abalam, the same demon Marcus had spoken to Louis about. Abalam agrees to release Nell from the possession but only if Marcus can remain silent for ten seconds and begins breaking Nell’s fingers as it counts out loud. After counting to three and breaking three of her fingers Marcus yells for Abalam to stop. During this time, Abalam tells Louis that Marcus wants a “blowing job.” Marcus realizes that a demon would know the actual name of the sex act and concludes that Nell is actually a very disturbed girl pretending to be possessed. Nell then tells them of Logan, the boy who supposedly impregnated her. Marcus arranges for Pastor Manley to come to the house to provide solace to the Sweetzers as Marcus prepares to leave. On their way home, Marcus and the film crew detour to the coffee shop where Logan works. Logan tells the crew that the only contact he had with Nell was a brief conversation six months ago at a party that was held at Pastor Manley’s home. Logan insinuates that he is gay and, therefore, would never have had sex with her. Marcus and the film crew leave the cafe and are driving out of town. They begin to question Nell’s story, and Marcus realizes that Pastor Manley was not truthful with him previously when he had said he had not seen Nell in an extended period of time. Marcus turns the van around and returns to the Sweetzer farmhouse. They enter the house to find numerous pentagrams and demonic symbols scrawled on the walls, but Nell and Louis are missing.
Marcus and the film crew wander into the woods, where they see a large fire and a congregation of hooded occultists led by Pastor Manley (Tony Bentley), the Sweetzers’ estranged Protestant minister. Louis is bound and gagged on a pole while hooded figures pray around an altar, which Nell is tied on top of. Marcus and the film crew watch as Nell gives birth to something that is not of the human race. Manley throws it into the fire, which causes the fire to grow rapidly while demonic roars emanate from within. At that moment, Marcus’ faith is resolved as he grabs his cross and rushes towards the fire in a frenzied attempt to combat the evil. Iris and Daniel are discovered and run away. Iris is tackled by a member of the occult congregation, who kills and dismembers her with an axe. Daniel tries to escape through the woods, but when he pauses to catch his breath, Caleb rises out of nowhere and decapitates him. The camera collapses to the ground, and the scene fades to black.