Father Gabriele Amorth, the Pope's personal exorcist, is an earthy man with a sense of humor who arrives in a quaint Italian village to address a case of alleged possession. Joined by the local priest, he enters a room where a man is tied up. During the exorcism, Amorth boldly names the spirit as Satan and forcefully sends it into a pig brought for the ritual. In a shocking turn of events, he then shoots the pig, landing himself in hot water with a Church tribunal for acting without authorization. Among the tribunal members is the friendly Bishop Lumumba, but there’s also the skeptical Cardinal Sullivan, who is doubtful about the legitimacy of exorcisms and demonic possession. Defending his actions, Amorth asserts that evil is real, explaining he merely staged a psychodrama to assist the troubled man. Feeling disgusted, he storms out of the tribunal.
Soon after, the Pope sends him to investigate a distressed young boy named Henry in Spain. Accompanied by his mother Julia and rebellious sister Amy, Henry travels from America to claim an old abbey—his father's bequest after a tragic car accident that left him mute. The family’s attempts to restore the abbey are interrupted by a foreboding fire. Spiraling into bizarre behavior, Henry is soon revealed to be possessed by a malicious entity. Desperate for help, he calls for a priest, but when local Father Esquibel arrives, he is met with an unexpected rejection, leading to Henry's infamous statement: "Wrong priest."
When Amorth arrives, he reluctantly enlists the untrained Esquibel to assist him, who acknowledges Amorth's reputation without having read his acclaimed works. Their initial efforts are comically misguided as Esquibel stumbles through the exorcism process. The situation escalates when Henry's demon begins to affect Amy, adding tension to the already fraught environment. In the course of their endeavors, Amorth learns that Julia has strayed from faith, yet he ultimately persuades her to pray, hoping for a miracle.
Meanwhile, disturbances in Rome force the Pope into hospitalization while he peruses documents relating to Henry’s case. Amorth uncovers a hidden well on the abbey grounds that leads to a sealed-off area inhabited by ancient malevolence. Discovering that a creator of the Spanish Inquisition had been possessed, he pieces together that those transgressions were hidden by the Church. He identifies Henry's demon as Asmodeus, a critical development for the exorcism.
As their connection deepens through the sharing of personal burdens—Amorth’s survivor's guilt from World War II and Esquibel's past indiscretions—the duo remains powerless against the demon. Their combined visions of haunting memories emphasize their failures, and Amorth’s willingness to sacrifice himself becomes the key to confronting his adversary. The power struggle takes a dark turn when Amorth attempts to take his life, but Asmodeus thwarts this, preferring a more insidious approach by trying to corrupt the sanctity of the Church itself.
With Esquibel’s assistance, Amorth finally repels the demon, conquering the dark manifestations tied to both men’s traumas. As the Pope recovers, so does Henry, victorious and rejuvenated. The two exorcists find themselves welcomed in Rome, where Lumumba replaces the disgruntled cardinal who fled to Guam. They gain access to exclusive Church archives, leading to their assignment of combating malevolent forces worldwide with the help of a treasure map uncovered by Amorth. With a twinkle in his eye, Amorth exclaims, "We're going to Hell!"
The film concludes with a poignant display of Amorth's legacy as a papal exorcist, noting his authorship of several well-regarded books. A humorous after-credits image of the real Amorth playfully sticking out his tongue encapsulates his vibrant spirit, alongside his birth and death dates.
