NOTE: This spoiler was submitted by Nash.
The film opens with Michael Kovak (Colin O’Donoghue) working for his father, Istvan Kovak’s (Rutger Hauer) undertaking business. Fed up with his life at home and working for his father, who won’t pay for his son to go to college, decides to join the seminary school. In his family one can only be a mortician or a priest. His plan is to abdicate his vows when he graduates, attaining a free college degree in the process.
After four years, Michael is ordained as a deaconate. However, after the ceremony, Michael writes a letter of resignation to his Father Superior, Father Matthew (Toby Jones), claiming a lack of faith as a reason for leaving. Father Matthew runs after Michael on the rainy streets trying to stop him, but he ends up tripping. A female bicyclist swerves out of the way and hits an oncoming car. Michael rushes over to her, but she is dying. The young woman sees Michael’s priest clothes and asks him for absolution. Michael comforts her and absolves her of her sins. Impressed by his handling of the situation, Father Matthew talks to Michael, believing it is his calling to become a priest and tries to convince Michael to stay. Father Matthew says he won’t send his resignation letter to his superiors. Father Matthew says that if Michael doesn’t complete his vows, his scholarship can be converted into a student loan. This would mean that Michael would owe them over 100,000 dollars. Father Matthew says they won’t do this as long as he accepts an invitation to travel to Rome in order to attend a two-month program on exorcism.
Michael attends the classes and is late for his first class. He is chastised by the teacher Father Xavier (Ciaran Hinds). Father Xavier goes over the way to prove and disprove actual demonic possessions from psychotics and schizophrenics. Michael meets a young woman named Angeline (Alice Braga) attending class who seems to be as skeptical about demonic possessions and religion as he is. She is a reporter covering the course for an article on exorcism in the newspaper. Father Xavier, who is a friend of Matthew and was asked to look after Michael and realizes that Michael is a skeptic, asks Michael to see a friend of his, Father Lucas (Anthony Hopkins). He is an expert exorcist with unorthodox methods.
Michael arrives at Father Lucas’ home and meets a pregnant sixteen year old girl named Rosaria there. She is one of the priest’s patients. We later find out that she was raped by her father. which led to her possession. Lucas introduces Michael to several of his patients and coaxes out the demons out of Rosaria, showing him what the spirits can do. Michael witnesses a demon taking over one girl but remains skeptical when it is all over. Michael expects something more, but Lucas says that the Devil is like a burglar in the home who doesn’t want you to know he’s there.
Michael sees Angeline out on the streets, and they go out for dinner. Angeline tells him that she is a reporter and that she wants to speak with Lucas who has denied all requests to be interviewed. Back in class, the power goes out and they end class early. Angeline and Michael do not talk. Michael voices his concerns about what Lucas is doing to Xavier but he tells him that Lucas gets results so do not worry.
Michael goes to Lucas’ house in order to question the Rosaria. He asks her whether she remembers things that the Devil does when she’s under his control. He pulls out a piece of jewelry that he found in his pocket after their session the other day. It is Rosaria’s but she does not remember giving it to him so he gives it back to her. She gets worse and worse and the demon completely takes over before she passes out. Michael is still skeptical and believes she is acting out because she clearly does not want the baby inside of her.
Lucas takes Michael to another patient of his, a young boy with bruises on his body. Lucas accuses the mother of hitting him. Lucas then looks him over and asks for the boy’s pillow. He pulls out a small frog/lizard from the pillow and claims it’s the devil. Lucas burns it and performs a ritual while Michael looks at the boy. The boy tells his mother that someone is going to die but we don’t hear who.
Lucas gets a call that Rosaria tried to drown herself. They arrive at the hospital and they can hear her screaming as they go down the hallway to her room. She is uncontrollable and there is nothing the doctors can do so Lucas performs another exorcism on the girl while Michael observes. The demon claims that Michael fears her but Michael still claims skepticism claiming that he can’t fear something that does not exist. Michael thinks she needs a shrink, not a priest. Lucas says that the Devil still has him fooled. Michael leaves but Lucas stays overnight outside the girl’s room. That night the baby dies from cardiac arrest and Rosaria dies from blood loss. Lucas feels he has failed her but Michael says he did what he thought was best. Lucas tells Michael to guard and defend his good soul as Michael leaves the hospital.
Michael decides to meet Angeline again and agrees to be interviewed. They talk about Rosaria and the entire event around her death. When Angeline asks her why he changed his mind about the interview, its because he needed to tell someone. Michael tells Angeline his story and Angeline tells him her’s. Her brother was put in an institution and told her that he had the devil inside her. He would tell her things the devil said that would turn out true.
That night, Xavier tells Michael that something has happened to his father. He is in a coma and has not regained consciousness. While Lucas shaves in front of his mirror, he purposely cuts himself and stares deeply into the mirror. Something has happened to him. When Michael returns to his room, he finds a large number of tiny frogs suddenly in his room. Michael checks into a hotel. Michael talks to his father on the phone who has regained consciousness. He is unable to leave for home because all the flights have been grounded due to a volcano. In the middle of the conversation, the line cuts out and its all of a sudden the doctor on the phone who tells him that his father passed away that afternoon.
We then find that Lucas has been acting very strangely and is obviously in the thralls of a demonic possession. Michael and Angeline go to Lucas’ house to find out what is going on but they find him sitting outside in the rain. They go into his house and Lucas regains enough control and lucidity to tell Michael that he has been possessed. Michael needs to find Xavier in order to perform the exorcism. Angeline and Michael look for Xavier but find that due to the volcano eruption and all flights being grounded, Xavier is stuck outside of the country.
Michael decides that it is up to him to force the demon out of Lucas. He performs the exorcism in a long drawn out battle with the demon constantly rebuking him but over the course of the exorcism, Michael regains his faith and forces the demon to reveal its name, Baal. The exorcism is completed and the demon leaves Lucas. Now with his faith intact, Michael leaves Rom and returns to America to bury his father.
The film ends with Father Michael Kovak entering a confessional and beginning to hear a young woman’s confession. He has decided to complete his vows and to remain a priest.