By many accounts, the low budget movie Equinox, originally shot in 1967, but not released until 1970, inspired the creative team responsible for Evil Dead quite a lot. Four young adults – Dave, Susan, Jim and Jim's girlfriend – head into the woods to look for a lost scientist, Dr. Arthur Waterman. The friends have a picnic and glimpse 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 the teenagers.
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. 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 Jim's girlfriend 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 prophecies 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.
With the end credits the film ends with "The End?" leaving the audience to wonder if the story will ever continue and if Susan is still possessed by Asmodeus.