Father Gabriele Amorth, the Pope's personal exorcist, and an earthy, humorous, practical man at times, arrives at a small village in Italy where a man is allegedly possessed by a spirit. Along with the local priest he enters the room where the man was tied up. While exorcising the man, he refers to the spirit as Satan and sends it into a pig which was brought in for the exorcism. As soon as the spirit enters the pig, Amorth shoots it with a shotgun.
This incident gets him in trouble with a Church tribunal, since he acted without permission from superiors. One tribunal member is a friendly African bishop, Lumumba, but one is a vicious American cardinal, Sullivan, skeptical of exorcism and even demonic possession. Amorth replies that evil does exist, and that he did not perform an exorcism, but rather, some psychological theater to help the mentally-disturbed man. Disgusted, Amorth walks out of the tribunal.
After this, the Pope assigns him to visit a possessed little boy named Henry in Spain. Henry, his mother Julia, and his rebellious teenage sister Amy travel to Spain from America to take possession of a mysterious old abbey which was Henry's father's sole bequest to his family after he died in a car accident, with Henry 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; brain tests show nothing abnormal.
Henry, satanically possessed, asks for a priest; the local Father Esquibel arrives, but Henry abuses him and says, "Wrong priest." Amorth arrives and enlists Esquibel as an assistant, though Esquibel is untrained as an exorcist. Esquibel says he has heard of Amorth, but hasn't read his books; Amorth says, "They're good books." Esquibel makes mistakes as a junior exorcist at first, including strangling Henry when Henry antagonizes him.
The duo make various attempts to exorcise Henry, without success. 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.
In Rome, the Pope is taken sick 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 finds that a founder of the Spanish Inquisition was possessed while being an exorcist, which let him infiltrate the Church and do many evils, including the Inquisition. Amorth also finds the Church covered this up, and eventually finds the name of Henry's demon, Asmodeus, which will assist the exorcism.
Amorth and Esquibel reveal to each other their own, and absolve each other's, sins or traumas: that Amorth has guilt from surviving World War II as an Italian partisan, and that a mentally-ill woman asked Amorth's help, and committed suicide when he did not help her enough; and Esquibel had extramarital intercourse with young women. While this helps them bond, they are still unable to exorcise Henry, and they have horrible visions of the women with whom they failed; the exorcism succeeds only when Amorth offers himself to be possessed, which chimes with Henry'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 Amorth and Esquibel. The Pope recovers, as does Henry.
The triumphant duo visit Rome, and find the skeptical American bishop has taken leave in Guam, and been replaced by Lumumba. Amorth and Esquibel are admitted to a special Church archive of mysterious events; Lumumba tells them they will be helping go to hundreds of other evil sites, with the help of a map Amorth discovered at the abbey, to combat the Devil. Amorth joyfully says, "We're going to Hell!" Finally, words on the screen narrate biographical details about Amorth's papal-exorcist career, including that he has written many books, and "The books are good."