Frederic Mason, Henry Czerny, a former priest, is desperate and praying for swift penance as he and his wife Ethel Mason, Mimi Kuzyk, mourn the recent death of their young daughter Joanie. His old colleague Father Graham, Nigel Bennett, worries that Frederic's faith is wavering, but Frederic insists the tragedy is God’s will and resolves to retreat to their isolated countryside home.
Their friend Doris, Kate Corbett, Joanie’s birth mother, visits and confirms she never told Joanie about this connection. That night, a young man with a sprained foot emerges from the woods and asks for help. He gives only his name, Aaron Smith, Mark O'Brien, and the couple, despite suspicion, invite him to stay. Aaron speaks at length about his adoption and his abusive past, and he laments a mysterious force that haunted him as a child before he collapses.
The next morning, Frederic finds Ethel cheerful, cooking breakfast while singing with Aaron, and she expresses gratitude to God that Aaron is there. Aaron later tells Frederic that he also suffers from mental lapses.
The following day, when alone with Frederic, Aaron begs him to kill him, or he swears to take Ethel from [him]. Frederic is angered, but Aaron collapses again. After Ethel is asleep, Aaron reveals to Frederic that he is his penance: during Frederic's prayers, Aaron became possessed and drove Joanie with his car. He wanders through the woods in search of Frederic, who is revealed to be Aaron’s biological father. Before meeting Ethel, Frederic had raped a woman in his former parish; she kept quiet to protect the church and died in childbirth unbeknownst to Frederic. Unable to feel love, Aaron says he becomes “possessed of something else.” Frederic refuses to believe him, and Aaron touches a flower, which instantly wilts.
Frederic encounters Graham at church and confesses, fearing that one sin might require another. Graham, quoting the Book of Revelation, explains that a sin does not cancel another, but committing one does not release any “ball of fire in the sky” either; he urges Frederic to distinguish between reality and the conscience he has created to punish himself.
That night, Aaron urges Frederic to kill him again, but Frederic continues to refuse, warning that a murder by a man of God could unleash catastrophe on humanity with “fire and fury.” Doris suddenly arrives and asks for a blessing because she has nightmares since Joanie's death. When Frederic returns with the Bible, Doris has been stabbed by Aaron, who threatens to kill her unless Frederic agrees to kill him. Frederic refuses and calls the police; Aaron then throws Doris against the wall to her death with a blow of his breath. Ethel collapses from bronchitis, and Frederic finally stabs Aaron, who dies in his arms. A police officer arrives as Frederic goes outside, where four balls of fire descend from the sky toward him.
