Four friends are attacked by a demon while on a picnic, due to possession of a tome of mystic information.

A newspaper reporter writing a story on psychiatric patient David Fielding watches him become violent after being shown a picture of his geology professor, Dr. Waterman. The reporter learns David's story from a tape recorded when the patient was admitted a year earlier. After receiving a frantic message from Dr. Waterman, David and three friends (Susan, Jim, and Vickie) arrive at the professor's demolished mountain cabin. Notified of the professor's disappearance, forest ranger Asmodeus promises to investigate. While exploring a cave, the youths are given an ancient book by an old man; later, it is snatched away by a crazed man, revealed to be Waterman. Although the professor dies when chased by David and Jim, his corpse mysteriously vanishes when the foursome are reunited. After Susan is attacked by Asmodeus, who flees when he sees her crucifix, the youths examine the old book and learn it is an occult bible, concerned with devil worship. After rescuing the girls from an ape-like monster, Jim separates from the rest and confronts Asmodeus. The ranger reveals himself to be the Devil and demands the book, but Jim flashes an occult symbol at him and escapes. Chased by another monster through an invisible barrier to a supernatural world, Jim is mesmerized and possessed by Asmodeus. David follows and retrieves the book, but learns that the professor's translations of the bible have uncovered the secrets of the supernatural world. Asmodeus kills the girls, who are fleeing with the book, and places a death curse on David. Neither the doctor nor the reporter believes David's story and suspect him of murdering his friends. As the reporter leaves the hospital, a supernatural being, disguised as one of David's dead friends, arrives to kill David.