Reverend Cotton Marcus, a charismatic but disillusioned evangelical minister in Louisiana, has performed numerous fake exorcisms for years. After a crisis of faith, he agrees to let a documentary crew follow him for one final exorcism—intending to expose exorcism as a hoax.

He is called to the rural Sweetzer farm, where Louis Sweetzer, a deeply religious farmer, believes his daughter Nell is possessed by a demon. Cotton initially treats it as a mental illness or abuse case and performs a theatrical "exorcism" to satisfy the family.

However, things take a darker turn when Nell begins exhibiting terrifying behavior—violent outbursts, speaking in tongues, and eerie knowledge of personal secrets. As Cotton and the crew delve deeper, they uncover disturbing secrets about Nell’s past, the family, and something far more sinister than they anticipated.