Sister Ann is a restless 25-year-old who devoutly believes that performing exorcisms is her calling. But she is at odds with the institution's traditions: nuns are not allowed to perform exorcisms, only priests. With the support of a mentor, a professor who senses her special gift, she is allowed to observe actual training sessions. Her desire to prove herself takes a personal turn when she meets one of the school's most disturbed patients. During their harrowing encounters, Sister Ann comes face to face with a demonic force that infests the school and has mysterious ties to her own past. It is then that the power of evil and her own startling abilities are fully realized.