A reporter visits David Fielding [Edward Connell] in a psychiatric hospital for a follow-up on the deaths of his three friends exactly one year and one day ago. David is largely catatonic, but he lashes out when shown a photo of Dr. Watermann [Fritz Leiber Jr.], his former professor, and in the scuffle he loses his cross, which sends him into a frenzy over its disappearance. The scene quickly shifts into a narrated investigation, as the reporter listens to tape recordings of the police interviews conducted soon after David was brought in, and pieces together the sequence of terrifying events that drove him to the edge of sanity.
The tale unfolds in a series of flashbacks that center on David Fielding, his companion Susan Turner [Barbara Hewitt], Jim Hudson [Frank Bonner], and Jim’s girlfriend, Vicki [Robin Christopher]. The four set out in search of Dr. Watermann in a remote forest canyon, where they discover that Watermann’s cabin has been utterly destroyed. Their search leads them into a cave, where they encounter an enigmatic old man [Louis Clayton] who offers them a battered, ancient book filled with magical lore and symbols. The book, Watermann’s notes describe as a veritable bible of evil, carries a sulfurous stink and even bears the Lord’s Prayer written backwards, hinting at experiments that push the boundaries between science and sorcery.
As the group delves deeper into Watermann’s arcane work, they learn that his bizarre experiments with the book’s ritual powers have gone disastrously wrong. Watermann’s efforts to summon demons spiraled out of control, culminating in the appearance of a giant tentacled creature that obliterated his house. In a frantic moment, Watermann himself appears and tries to snatch the book back; a chase ensues, and David tackles him, causing Watermann to strike his own head and die. Jim and David escape, but Watermann’s body vanishes in a supernatural fashion, hinting at forces far beyond ordinary science.
Disguised as a forest ranger, Asmodeus [Jack Woods] learns that Jim and David possess the book, and he moves quickly to recover it by conjuring two nightmarish monsters: a colossal ape-like creature and a green-skinned, fur-clad giant—their purpose is clear: retrieve the book at all costs. The ape-like creature ruthlessly kills the old man from the cave, underscoring the deadly reach of the demon’s reach and the peril that now engulfs the group.
A darker current runs through the story as Asmodeus attempts to assert control in more intimate ways. He tries to sexually assault Susan, but the protective cross repels him. In a chilling turn, Susan loses the cross, and she becomes demon-possessed, turning her fear against Vicki, who cannot stop the possessed woman from attacking. David intervenes, drawing a mystical symbol from the book which causes Susan to lose consciousness and buys them a sliver of time.
The stakes escalate when Asmodeus murders Jim, then pretends to be him in a cruel ruse that deceives David for a while. The ruse collapses as David realizes the impostor’s deception, and a furious struggle ensues, culminating in Asmodeus revealing his true form: a winged, red demon. The carnage continues as Asmodeus kills Vicki and then launches an onslaught against David and Susan. The couple flees to a cemetery and seeks shelter behind a towering stone cross atop a grave.
In a cataclysmic moment, the demon flies into the cross and dies, and the cemetery erupts in flames, destroying Susan in the blaze. A towering, shadowy figure prophesies a grim fate: David will be dead in one year and one day. Panicked, David hurls himself onto a nearby road, where a driverless car runs him down. A car with two passengers stops to help him, and a sense of fragile mercy interrupts the otherwise relentless doom.
Despite the rescue, David’s psyche never fully heals. He is ultimately confined to a mental hospital, living with the creeping dread of what he has endured. One year and one day later, a redesigned horror resurfaces when an evil-faced, reanimated Susan arrives at the hospital, bent on finishing what she and the dark forces began. The film closes on a note of fatalistic dread, leaving the fate of David Fielding [Edward Connell] firmly locked in the shadow of the supernatural, the trauma of memory, and the unshakable belief that the past can never be fully exorcised.
