Though he is filled with doubt about the subject, seminary student Michael Kovak (Colin O'Donoghue) finds he must attend a Vatican school of exorcism. Believing possession to be the work of a troubled mind rather than actual demons, hel urges his superiors to look for answers in psychiatry. Then Michael becomes the apprentice of Father Lucas (Anthony Hopkins), a veteran exorcist, and encounters a terrifying force that causes him to question everything he believes.
