For their vacation, Ash, his wife Linda, their friend Scotty, and his wife Shelly travel to an apparently abandoned cabin in the woods. In the cabin, the quartet finds a recorder, audio reels, and an old book. Scotty plays one of the reels, which were made by the cabin's owners, Professor Raymond Knowby, and his wife Henrietta. The reel reveals the professor and his wife were working on translating the book, a seemingly demonic sex guide known as the Necronomicum Ex-Mortis. While reciting a mantra he found in the Necronomicum, Knowby seemingly went insane, laughing maniacally as he repeated the words over and over again, summoning an evil force that shatters one of the cabin's windows.
The group ignores the broken window, and while Shelly goes to lie down, Ash, Linda and Scotty have athreesome in a bedroom. Shelly walks in on the three, and distraught over Scotty's infidelity, runs out into the forest, where she is violated by possessed trees, turning her into a "Deadite". Shelly returns to the cabin, knocks Ash out, and stabs Linda with a piece of bark she pulls out of her vagina, infecting the other woman with the evil. Shelly proceeds to seduce Scotty, have sex with him, and rip his throat and testicles out.
When Shelly tries to attack Linda, Ash fends her off with a fireplace poker, and locks her in the cellar, but she escapes, and begins having sex with the now demonic Linda. Aroused by the sight of the two Deadites going at each other, Ash tries fingering Shelly, causing his hand to become possessed. Ash is forced to cut his hand off with a chainsaw, which he then uses to kill Shelly by forcing her "ride" the blade.
Linda, who has returned to normal, confronts Ash with a locket identical to the one he had earlier given her. Inside the locket is a picture of Ash having sex with another woman, which angers Linda to the point that she becomes a Deadite again. Ash and Linda wind up having sex, and afterward Ash beheads Linda with the chainsaw, and takes her body out into the woods to dispose of it. Linda's still-living severed head taunts Ash, annoying him to the point that he masturbates onto it out of spite before burying it.
Ash returns to the cabin, and goes into hysterics when the furniture and fixtures (including a mounted deer head) go haywire. Outside, Linda (who has reattached her head) rises from the grave, along with Deadite versions of two of Ash's ex-girlfriends. Ash tries to kill the Deadites with the chainsaw (which he has replaced his missing hand with) but it will not start, allowing the demonesses to force him into an orgy, which ends when Ash grabs the chainsaw, and dismembers all three women with it.
Sounds like a winner!!