After being pulled through a time portal, Ash Williams lands in A.D. 1300, where he is soon 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 (as he knew Henry was innocent, and his persecution was simply a witch hunt) and killing a Deadite in full view of everyone, Ash is celebrated as a hero. He also grows attracted to Sheila, the sister of one of Arthur's fallen knights.
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. As he enters a haunted forest, an unseen force pursues Ash through the woods. Fleeing, he 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 clone of Ash and attack him, after which Ash kills 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 safelyÂ â "Klaatu barada nikto". 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, his 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 later transformed into a Deadite. Ash becomes determined to lead the humans against the army of the dead. Reluctantly, the people agree to join Ash. Using scientific knowledge from textbooks in the trunk of his 1973 Oldsmobile Delta 88, and enlisting the help of Duke Henry, Ash successfully leads the medieval soldiers to victory over the Deadites and Evil Ash, saving Sheila and bringing peace between Arthur and Henry in the process. The Wise Men return him to his own time, giving him a potion to drink after reciting the magic phrase.
Back in the present, Ash recounts his story to a fellow employee at his job, working in housewares at a store called "S-Mart". As he talks to a girl who is interested in his story, a surviving deadite, allowed to come to the present due to Ash again forgetting the last word of the magic phrase, attacks the customers. Ash attacks and kills it using a Winchester rifle from the store's Sporting Goods department, finally ending the deadite threat.

Question: What does Ash need to get to go back home?
Answer: The Necronomicon Ex-Mortis.
Alternative answer: A book with magical powers.