Four young adults – David Fielding, Susan Turner, Jim Hudson, and Jim’s girlfriend, Vicki, head into the woods to look for a lost scientist, Dr. Arthur Waterman. They have a picnic and see a mysterious castle in the woods. They find that Dr. Waterman’s cabin seems to have been destroyed. A forest ranger, who is Asmodeus in human form, watches over them. When the group stumbles into a cave, a strange old man presents them with an ancient book filled with magical lore and symbols. Asmodeus sends monsters – a giant ape-like creature and a green-skinned, fur-clad giant – to retrieve the book from them at all costs. The ape-like creature kills the old man. Reading through the book, David learns that it belonged to Dr. Waterman, who had experimented with its demon-summoning rituals. He lost control of the giant tentacled creature he conjured, however, which destroyed his house. The castle seems to have disappeared; however, the friends discover that it has been rendered invisible by magic.
After killing Jim, Asmodeus reveals his true form, that of a winged red demon. Asmodeus kills Vicki and then attacks Dave and Susan. Dave and Susan flee to a cemetery and destroy the demon with a cross. As it dies, the cemetery explodes, killing Susan. Dave sees a shadowy giant who prophesies that Dave will be dead in one year and one day. Dave loses his sanity, and is confined to a mental hospital. One year and one day later, an evil-faced Susan arrives at the hospital to visit him. The film concludes with end credits saying “The End?”.