A reporter visits David Fielding in a psychiatric hospital for a follow-up story on the deaths of his three friends exactly one year and one day ago. David is catatonic but attacks the reporter when shown a photo of Dr. Watermann, his former professor. In the scuffle, David loses his cross and grows frantic over its disappearance. The reporter listens to tape recordings of the police interviewing David soon after he was brought to the hospital and reconstructs the events — told in flashback — which drove him insane.
David, Susan Turner, Jim Hudson and his girlfriend, Vicki, search for Dr. Watermann in a forest canyon and find that his cabin has been destroyed. While exploring a cave, they encounter a cackling old man who gives them an ancient book filled with magical lore and symbols. The book, which Watermann's notes describe as a "veritable bible of evil," reeks of sulfur and contains the Lord's Prayer written backwards. The group learns Watermann's experiments with the book's demon-summoning rituals went awry. When he lost control of the giant tentacled creature he conjured, it destroyed his house.
Dr. Watermann suddenly appears and snatches the book from David; he and Jim give chase. David tackles him, causing him to strike his head and die. After Jim and David leave, Watermann's body supernaturally vanishes. A forest ranger, Asmodeous, discovers Jim and David have the book and sends monsters – a giant ape-like creature and a green-skinned, fur-clad giant – to retrieve it. The ape-like creature kills the old man from the cave.
Asmodeus starts to sexually assault Susan, but her cross repels him. After Susan accidentally loses the cross, she appears demon-possessed and attacks Vicki, who is unable to stop her. David arrives and displays a mystical symbol from the book, causing Susan to lose consciousness.
Asmodeus kills Jim and reveals his true form: a winged, red demon. After killing Vicki, Asmodeus attacks David and Susan, who flee to a cemetery and cower behind a large stone cross atop a grave. As the demon flies into the cross and dies, the cemetery erupts in flames, killing Susan. A giant shadowy figure prophesies that David will be dead in one year and one day. When David panics and flees to a nearby road to summon help, a driverless car runs him down. Another car with two passengers stops to help him.
David loses his sanity and is confined to a mental hospital. One year and one day later, an evil-faced, reanimated Susan arrives at the hospital to kill him.
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