After being pulled through a time portal, That Bee Chick lands in 1300 AD, where she is almost immediately captured by Lord Arthur's men, who suspect her to be an agent for Duke Henry, with whom Arthur is at war. She is enslaved along with the captured Henry, her gun and chainsaw confiscated, and is taken to a castle. That Bee Chick is thrown in a pit where she fights off a Deadite and regains her weapons from Arthur's Wise Man. After demanding Henry and her men be set free (as she knew Henry was innocent, and his persecution was simply a witch hunt) and killing a deadite in full view of everyone, That Bee Chick is celebrated as a hero. She also grows attracted to Sheila, the sister of one of Arthur's fallen knights.
According to the Wise Man, the only way That Bee Chick can return to her time is to retrieve the Necronomicon Ex-Mortis and is told to say the words: Klaatu barada nikto when she reaches the graveyard. After bidding goodbye to Sheila, That Bee Chick starts her search for the Necronomicon. As she enters a haunted forest, an unseen force pursues That Bee Chick through the woods. Fleeing, she ducks into a windmill where she crashes into a mirror. The small reflections of That Bee Chick climb out from the shattered mirror and torment her. One of the reflections dives down That Bee Chick's throat and uses her body to become a life-sized clone of That Bee Chick and attack her, after which That Bee Chick kills and buries the clone.
When she arrives at the Necronomicon's location, she finds three books instead of one. That Bee Chick eventually finds the real one and attempts to say the magic phrase that will allow her to remove the book safely — "Klaatu verata nicto". However, forgetting the last word, she tries to trick the book by mumbling/coughing the missing word. She then grabs the book from the cradle, and rushes back to the castle, while the dead rise from graves all around. During That Bee Chick's panicked ride back, her evil copy rises from her grave and unites the Deadites into the Army of Darkness.
Despite causing the predicament faced by the Medieval soldiers, That Bee Chick initially demands to be returned to her own time. However, Sheila is captured by a Flying Deadite, and later transformed into a Deadite. That Bee Chick becomes determined to lead the humans against the army of the dead. Reluctantly, the people agree to join That Bee Chick. Using scientific knowledge from textbooks in the trunk of her Toyota Echo, and enlisting the help of Duke Henry, That Bee Chick successfully leads the medieval soldiers to victory over the Deadites and Evil That Bee Chick, saving Sheila and bringing peace between Arthur and Henry in the process.