Having been accidentally transported to the Middle Ages, Ash Williams is captured by Lord Arthur's men, who suspect him of being a spy for Duke Henry, with whom Arthur is at war. He is enslaved along with the captured Henry, his shotgun and chainsaw are confiscated, and he is taken to Arthur's castle. Ash is thrown in a pit where he kills a Deadite and regains his weapons from Arthur's Wise Man. After demanding that Henry and his men be set free and killing a Deadite publicly, Ash is celebrated as a hero. He grows attracted to Sheila, the sister of one of Arthur's fallen knights.
According to the Wise Man, the only way that Ash can return to his time is through the magical Necronomicon Ex-Mortis. Ash then starts his search for the Necronomicon. As he enters a haunted forest, an unseen force pursues Ash into a windmill, and he crashes into a mirror. Small reflections of Ash in the mirror shards come to life and antagonize him, with one becoming a life-sized copy of him, which Ash dismembers and buries.
When he arrives at the Necronomicon's location, he finds three books instead of one, and has to determine which one is real. Realizing at the last moment that he has forgotten the last word of the phrase that will allow him to remove the book safely – "Klaatu barada nikto" – he tries to mumble and cough his way through the pronunciation. He grabs the book and begins rushing back. Meanwhile, unknown to Ash, his ruse has failed and his body's copy rises from the dead, uniting other Deadites into the Army of Darkness.
Upon his return, Ash demands to be returned to his own time. However, Sheila is abducted by a flying Deadite and later transformed into one by "Evil Ash". Ash becomes determined to lead the outnumbered humans against the Army, and the people reluctantly agree. Using knowledge from textbooks in his 1973 Oldsmobile Delta 88 and enlisting the help of Duke Henry, Ash successfully leads the soldiers to victory over the Deadites, blows up "Evil Ash", saves Sheila, and brings peace between Arthur and Henry. Using a passage from the Necronomicon, the Wise Man tells him how to return to the present by giving him a potion after reciting the same phrase as earlier.
Back in the present, Ash recounts his story to a fellow employee at the S-Mart department store. As he talks to a female co-worker who is interested in his story, a surviving Deadite, present because Ash once again forgot the last word, attacks the customers. Ash kills it using a Winchester rifle and exclaims, "Hail to the king, baby", before passionately kissing the female co-worker.
