In 1987, Amorth, the Pope’s personal exorcist, an earthy, scooter-riding, humorous, practical man, visits an Italian village where a man is seemingly possessed by a demon. With the local priest, Amorth enters the room where the man is tied up. While exorcising him, using a Saint Benedict Medal sacramental, Amorth drives the demon into a pig, which is then killed with a shotgun.

This incident gets Amorth in trouble with a Church tribunal, since he acted without permission from superiors. One tribunal member is a friendly African bishop, Lumumba, but another is a vicious American cardinal, Sullivan, skeptical of demonic possession. Amorth replies that evil does exist, and that he did not perform a true exorcism, but rather, some psychological theater to help the mentally-disturbed man. Disgusted, Amorth walks out of the tribunal.

The Pope assigns him to visit a possessed boy named Henry in Spain. Henry, his mother Julia, and his rebellious teenage sister Amy had traveled from America to take possession of a mysterious old Spanish abbey which was Henry’s father’s sole bequest to his family after he died in a car accident in which Henry was also present. The traumatized Henry has not spoken since the accident. Workmen, who were restoring the abbey so the family could sell it, leave after a sinister fire. Henry starts behaving bizarrely; serum chemistry panels and MRI show nothing abnormal.

Henry, diabolically possessed, requests a priest; the local Father Esquibel arrives, but Henry obscenely derides him. Amorth arrives and enlists Esquibel as assistant, though Esquibel is untrained as an exorcist. Esquibel says he has heard of Amorth but not read Amorth’s books; Amorth says, “They’re good books.” Amorth advocates the importance of prayer, though Esquibel makes mistakes as an assistant exorcist, including strangling Henry when Henry antagonizes him, mocking him for his sins.

The duo attempt to exorcise Henry, without success, as he utters blasphemous phrases during the rite. Henry’s demon even possesses Amy at times. Amorth finds Julia has not been a religious believer since childhood, but he convinces her to pray after she reveals she believed her guardian angel helped her in childhood.

In Rome, the Pope becomes ill while reading documents about the Spanish case, and is hospitalized. Amorth finds a well on the abbey grounds going down to a complex sealed off by the Church as demonically dangerous. He learns that a founder of the Spanish Inquisition, an exorcist, was possessed, which let him infiltrate the Church and do many evils. Amorth also finds the Church covered this up, and eventually discovers the name of Henry’s demon, Asmodeus, which will assist the exorcism.

Amorth and Esquibel participate in the sacrament of Confession and Absolution, mutually confessing, and absolving each other of, their sins: that after Amorth, an Italian partisan, survived World War II and vowed to serve God in gratitude, a mentally-ill woman asked for Amorth’s help, and died by suicide when he did not help her due to pride; and Esquibel fornicated with a young woman he did not later marry. The two ready themselves; Amorth instructs Esquibel to wear a Miraculous Medal necklace. During the exorcism, they have horrible visions of the women whom they failed. The exorcism succeeds only when Amorth offers himself to be possessed, which chimes with Asmodeus’s previously stating that he wants to destroy Amorth.

Amorth tries to hang himself, but the demon doesn’t allow it, preferring that Amorth infiltrate and destroy the Church. However, Esquibel helps Amorth drive away the demon, and demonic appearances resembling the two women who troubled the men. The Pope recovers, as does Henry.

The triumphant duo visit Rome, and find Sullivan has taken leave in Guam, being replaced by Lumumba. Amorth and Esquibel are admitted to a special Church archive; Lumumba tells them they will be visiting hundreds of other evil sites, with the help of a map Amorth discovered at the abbey, to combat the Devil. Amorth jokingly says, “We’re going to Hell!” Finally, words on the screen narrate details about Amorth’s papal-exorcist career, including his writing many books, and “The books are good.”

An after-credits photo of the real Amorth sticking out his tongue appears, along with his birth and death dates.