Also: Timothy Patrick Quill as Blacksmith, Michael Earl Reid as Gold Tooth, Bridget Fonda as Linda, Patricia Tallman as Possessed witch, Angela Featherstone as S-Mart store girl, Noah Gillespie as Pit monster
After a brief flashback to Evil Dead II, which explains the Necronomicon Ex-Mortis and how Ash Williams got to where he is, Ash is pulled through a time portal and lands in Medieval England 1300 AD, where he is almost immediately captured by Lord Arthur's men, who suspect him to be an agent for Duke Henry, with whom Arthur is at war.
He is enslaved along with the captured Henry, his gun and chainsaw confiscated, and is taken to a castle. Ash is thrown in a pit where he fights off a Deadite and regains his weapons from Arthur's Wise Man. He knew Henry was innocent, and his persecution was simply a witch hunt (Henry stood up for him, and swore he did not know who Ash was).
After demanding Henry and his men be set free, and killing a deadite in full view of everyone, Ash is celebrated as a hero. He also grows attracted to the sister of one of Arthur's fallen knights, Sheila. According to the Wise Man, the only way Ash can return to his time is to retrieve the Necronomicon Ex-Mortis, a book with magical powers.
After bidding goodbye to Sheila, Ash starts his search for the Necronomicon. Entering a haunted forest, an unseen force pursues Ash through the woods. Fleeing, Ash ducks into a windmill where he crashes into a mirror. The small reflections of Ash climb out from the shattered mirror and torment him.
One of the reflections dives down Ash's throat and uses his body to become a life-sized copy of Ash, after which Ash kills him and buries the clone. When he arrives at the Necronomicon's location, he finds three books instead of one.
Ash eventually finds the real one and attempts to say the magic phrase that will allow him to remove the book safely "Klaatu barada nikto". (The line was taken from the 1951 film The Day the Earth Stood Still). However, forgetting the last word, he tries to trick the book by mumbling and coughing the missing word.
He then grabs the book from the cradle, and rushes back to the castle, while the dead rise from graves all around. During Ash's panicked ride back, Ash's evil copy rises from his grave and unites the Deadites into the Army of Darkness. Despite causing the predicament faced by the Medieval soldiers, Ash initially demands to be returned to his own time.
However, Sheila is captured by a Flying Deadite, and then transformed into a Deadite. Ash becomes determined to lead the humans against the skeletal Deadite army. Reluctantly, the people agree to join Ash. Using scientific knowledge from textbooks in the trunk of his 1973 Oldsmobile Delta 88, he enlists the help of Duke Henry.
Ash successfully leads the Medieval soldiers to defeat his Deadite clone, Evil Ash and his Deadite Army, saves Sheila, and brings peace between Arthur and Henry in the process. After this, the Wise Men return him to his own time using a potion drink made from the Necronomicon and reciting the magic phrase.
Back in the present, Ash is at the housewares department of S-Mart store telling a male co-worker (Ted Raimi) all about his adventure back in time, and how he could have been king. As he talks to a girl who is interested in his story, a surviving Deadite is allowed to come to the present due to Ash again forgetting the last word of the magic phrase. The Deadite starts wreaking havoc in the store, attacking the customers.
Ash attacks and kills it using weaponry found in the store, finally ending the deadite threat. The film ends with Ash in voice over saying, "Sure, I could have stayed in the past. Could have even been king. But in my own way, I am king." He then says out loud, while kissing the attractive female co-worker whose life he has just saved, "Hail to the King, baby!"