In early-1950s St. Louis, the strange case of the eleven-year-old boy, Robbie Mannheim, catches the eye of the troubled former World War II veteran and now a Catholic priest, Father William Bowdern. Bearing alarming markings on his little body, while exhibiting a disturbing behaviour and an incomprehensible power after the death of his Aunt Hanna, Robbie's worried parents turn to modern medicine to help their child; however, when science fails, they resort to religion. Now, with the Vatican's reluctant consent, Bowdern must face the unholy entity that possesses Robbie and perform the first documented exorcism in America. But, the dark enemy is powerful, and he'll use Father William's inner fears against him. Can the conflicted priest cast out the demon, and save not only the boy's life but also his very soul?