After a brief flashback to Evil Dead II, which explains the Necronomicon and how Ash got to where he is, Ash lands in Medieval England, 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. After demanding Henry and his men be set free, Ash is celebrated as a hero, and 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. 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 torture 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 him.
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 send him home. However, forgetting the last word he tries to trick the book by mumbling the missing word, but that unleashes the Evil Dead. Ash simply grabs the book and rushes back to the castle, while the dead rise from graves all around. During Ash's panicked ride back, Ash's 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, plus enlisting the help of Duke Henry, Ash successfully leads the Medieval soldiers to defeat his Deadite clone, Evil Ash, and his Deadite Army and save Sheila. After this, he is brought back to his own time using a potion made from the Necronomicon.