Ash Williams and his girlfriend Linda travel to an abandoned cabin in the mountains for a romantic vacation. Ash plays a reel-to-reel tape recorder containing translations of the Necronomicon Ex-Mortis recited by Professor Raymond Knowby. The ancient incantation unleashes an evil forest spirit that possesses Linda. Ash is forced to decapitate Linda with a shovel and bury her outside, but the evil entity quickly sweeps through the woods and infects Ash, knocking him unconscious.

Ash revives at daybreak, temporarily cleansed by the sunlight, but finds the bridge back to safety completely wrecked. Retreating to the cabin as night falls, Linda's headless corpse attacks him, and her severed head bites deeply into his right hand. Ash severs his own hand with a chainsaw after the limb becomes possessed and tries to kill him. Meanwhile, the professor's daughter, Annie Knowby, arrives near the cabin with her research partner Ed Getley, guided by locals Jake and Bobbie Joe.

Finding Ash covered in blood, the newcomers assume he murdered the professor and lock him in the fruit cellar. There, Ash is attacked by the rotting Deadite corpse of Henrietta Knowby. Once Annie realizes the evil presence is real, she frees Ash. The cabin erupts into supernatural violence: Ed turns into a demon and Ash hacks him apart, Bobbie Joe flees into the woods and is killed by trees, and Jake tries to force the survivors outside at gunpoint before getting dragged into the cellar by Henrietta. Ash mounts the chainsaw onto his severed stump, saws off his shotgun, and descends into the basement to obliterate Henrietta. Annie recites a Sumerian incantation from missing pages of the book to open a temporal vortex to banish the demon. A dying hand stabs Annie, but the portal pulls the giant Kandarian Demon away along with Ash and his car.

Ash is sucked through the swirling sky portal and dropped into a medieval desert landscape in the year 1300 AD. He is immediately surrounded by a squad of armed knights on horseback who prepare to execute him. Suddenly, a flying winged demon appears, which Ash promptly blasts out of the sky with his shotgun. Astonished by his weapon, the knights bow down and hail Ash as the savior sent to deliver them from the Deadites. Ash drops to his knees in horror and screams as he realizes he is stranded in the Middle Ages, setting up the events of Army of Darkness.