Reverend Cotton Marcus, a disillusioned preacher, invites a film crew to document his final exorcism, aiming to expose the deceit behind his ministry. Setting off to rural Louisiana, he meets Louis Sweetzer, a devout farmer who believes his teenage daughter, Nell, is possessed by a demon. Initially skeptical, Cotton performs a staged exorcism to comfort the family, using tricks and props to simulate an expulsion. However, events quickly take a disturbing turn when Nell begins exhibiting increasingly violent and inexplicable behavior, suggesting that her condition might not be as easily dismissed or fabricated as Cotton thought.

As Cotton and the film crew delve deeper, they uncover troubling evidence pointing to a dark and sinister force at play. Old secrets emerge, casting doubt on their understanding of what’s happening to Nell. The line between reality and superstition blurs, challenging Cotton's faith and skepticism. Struggling to protect Nell, the team faces escalating terror and finds themselves confronting questions of evil that they are ill-prepared to answer. With the tension mounting, the true nature of Nell’s affliction threatens to unravel everything Cotton has ever believed.