Having attended seminary school in order to get a free college education, mortician's son Michael Kovak (Colin O'Donoghue) is all set to renounce his vows and leave priesthood behind. Convinced that Michael's true calling lies with the Catholic Church, however, his superior, Father Matthew (Toby Jones), persuades him to travel to Rome to attend a series of lessons on exorcism.
Won over by a cash incentive, Michael agrees but soon finds his scepticism deepening as he sits through Father Xavier's (Hinds) classes. Perturbed by his student's cynicism, Xavier sends Michael to meet Welsh priest Father Lucas (Anthony Hopkins), a veteran exorcist presently involved in the case of a young pregnant woman said to be possessed by demons...
Despite witnessing some seemingly inexplicable incidents, Michael is not fully convinced and finds himself torn between rationality and faith. As Father Lucas becomes overwhelmed by his work, however, Michael must come face-to-face with his worst fears.