The Reverend Cotton Marcus (Patrick Fabian) lives in Baton Rouge, Louisiana, with his wife (Shanna Forrestall) and son (Justin Shafer). Marcus is accustomed to performing exorcisms on "possessed" individuals, but his faith has waned after reading of an autistic child being killed during an exorcism, reminding him of his own disabled son. He comes to realize that he attributes the healing of his son 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 a plea letter from an individual seeking an exorcism leading him to the farmhouse of Louis Sweetzer (Louis Herthum), a man who claims to have a possessed daughter named Nell (Ashley Bell). 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 by a powerful demon Abalam.
That night, Nell mysteriously appears in Marcus' hotel room. The team tries to contact Louis to get permission to film her, to no avail. Wishing to prove his point, Marcus admits Nell to the hospital for testing, hoping to receive 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. 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 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. As they try to confront her, she heads 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 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 was shown battling in the other picture, Iris hacked to pieces with an axe, and Dan decapitated.
Louis comes home and listens to 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, noises are heard upstairs. They discover Nell has climbed atop her dresser. As they try calming 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 her porch. While approaching her, Nell tackles Marcus, attempting to harm him. The struggle leads inside as Louis chases them with a shotgun. Attempting to keep Louis from killing Nell, Marcus agrees to attempt another exorcism.
They relocate to the barn where Nell killed the cat. The demon takes Nell again, making her bend backwards and break two of her fingers. She tells the crew that Nell is "in the fire", and that Louis will soon join her. "Nell" asks Marcus to let her give him a 'blowing' job. Marcus informs that this is not the correct term for such thing and therefore believes Nell is not possessed. Upon confronting Nell with this, she breaks down and admits to having sex with a boy named Logan and committing her violent acts out of shame of her pregnancy.
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; he insinuates that he is gay and, therefore, would never have had sex with her. Marcus and the film crew wonder why Nell would lie, and Marcus immediately realises one of the things the devil is known as is "The prince of lies". The film crew follows as he returns to the Sweetzer's farmhouse. They enter the house to find numerous pentagrams and demonic symbols scrawled on the walls, but Nell and her family 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 Sweetzer's estranged Protestant minister. Nell's father 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 voices grow louder. 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 murders her with an axe. When Daniel pauses to catch his breath, Caleb rises out of nowhere and mortally wounds him. The camera falls to the ground and the scene fades to black.