NOTE: This spoiler was submitted by Emilio

The film opens with a prompt saying that due to a rise in demonic possessions around the world, the Catholic Church has begun allowing schools outside of Rome to train priests to perform exorcisms.

In the opening scene, a little girl named Ann (Debora Zhecheva) is in her room with the door locked. Suddenly, her mother knocks on the door and wants to be let in. Ann ignores her and leaves the door locked. Ann’s mother continues to plead with her to open the door. Her mother begins violently banging against the door to try to get in. Finally, we see Ann’s mother is banging her head against the door, clearly possessed by something.

Years later, an adult Ann (Jacqueline Byers), has become a nun who is currently on assignment, assisting at St. Michael the Archangel School of Exorcism in Boston. Sister Ann helps in the medical ward, attending to patients’ daily needs (bathing, feeding, etc.). One day, she befriends a young girl, a patient named Natalie (Posy Taylor).

Ann is curious about the ways of exorcism and occasionally sneaks into an exorcism class taught by Father Quinn (Colin Salmon). She is later admonished because nuns are prohibited from being taught the ways of exorcisms. Priests can only perform the act of exorcism.

One afternoon, Ann gives a sponge bath to a patient when he suddenly attacks her. He begins speaking in a different voice and says something that reminds Ann of what her mother used to say when she was possessed. The entire incident is caught on a security camera. Father Quinn is convinced that this demonic spirit will continue to target Ann and that she should know how to defend herself. After Father Quinn convinces the Cardinal, Ann is allowed to attend exorcism classes.

Father Quinn takes the students to see an exorcism in person during her first class. The possessed individual is revealed to be Natalie. Two students, Father Dante and Father Raymond are selected to try their hand at leading the exorcism. When it goes awry, Father Quinn tries to exorcise the demon from Natalie, but she gets the upper hand. Ann eventually runs into the room and can appeal to Natalie, and the demonic spirit leaves. Later, Ann is informed that the demonic entity merely retreated because it felt threatened, thus making the exorcism unsuccessful. Natalie is informed to be a terminal case.

As Ann continues to be plagued by the same demonic spirit, she looks into records of individuals that were considered terminal cases. With Father Dante’s help, Ann can access the hidden records of those patients, only finding one woman who could survive. Still, her face is heavily scarred as a result of the exorcism. All the other patients died.

Father Dante later reveals to Ann that he believes his pregnant sister is being possessed and wants Ann to lead an unauthorized exorcism. Since she had some success with Natalie, Ann decided to help. After an intense session, Ann can release the demon’s hold on Dante’s sister.

In a private moment afterward, Ann tells Dante that she became pregnant with a little girl before she became a nun. Instead of choosing to raise her daughter, Ann gave her up for adoption right after the baby was born.

Ann is called into a meeting with Father Quinn and the Cardinal the next day. They reveal to Ann that shortly after their session with Father Dante’s sister, she committed suicide. As a result, Ann chooses to return to her monastery, St. Mary’s.

Father Dante later goes to see Ann at St. Mary’s to tell her that Natalie was released, but shortly afterward, Natalie experiences a relapse and is transported by ambulance back to the hospital. But, when the ambulance arrived, the doors were opened to reveal that Natalie had killed everyone inside the vehicle. Father Dante gives Ann a necklace that Natalie is holding in her hand. Ann looks at the necklace and tells Dante that this necklace is the same one she gave her daughter when she was born, thus revealing that Natalie is Ann’s daughter.

Dante convinces Ann to help lead another unauthorized exorcism. So, he sneaks Ann into the catacombs under St. Michael’s, where Natalie is being held. Natalie eventually breaks out and kills several priests, one of which is Father Raymond. After a struggle, Ann convinces the demonic spirit to release Natalie and take her instead. So, the spirit leaves Natalie, who hides with Father Dante, and goes into Sister Ann.

Ann becomes possessed and begins hunting Natalie in the catacombs. Ann dispatches Dante, leaving him unconscious. As Natalie hides near a fountain filled with holy water, the possessed Ann attacks her. Ann’s normal self eventually comes to and wrestles with the demon inside of her. As she physically battles with the spirit, her body falls into the fountain, and the holy water purges the spirit out of Ann’s body. Ann is rescued from the water and reunites with Natalie and Dante.

Ann recovers in the hospital but can say goodbye to Natalie. Natalie reunites with her adopted mother as Ann looks on. As a result of what happened, Ann is rewarded with an academic scholarship to study at the Vatican. As she says her goodbyes to Father Quinn, he reminds her that he knows who she is now that she has battled with the devil.

On her way to the airport in a cab, Ann spots a woman staring at her on the street. The woman eventually appears beside Ann’s window, sporting white eyes. Ann looks in front and sees that the cab driver is no longer in the driver’s seat. He is now beside her in the backseat. Ann takes out her crucifix and puts it to the cab driver’s face, ready to do battle again. The cab driver lunges at Ann as the film cuts to black.