Thadeous and Fabious are sons of King Tallious in the Kingdom of Mourne. They are warriors: Fabious is dashing and skilled and Thadeous is lazy and ineffectual. While celebrating his latest victory over the evil sorcerer, Leezar, who has been ravaging Tallious's kingdom, Fabious introduces the virgin Belladonna whom he has freed from a tower and wishes to marry. Though his brother makes him the best man, Thadeous skips the wedding after overhearing Fabious' Knights Elite, led by Boremont, talk about him negatively. The wedding is then crashed by Leezar, who reveals himself to be the one who placed Belladonna in the tower. Leezar re-kidnaps her and flees. Returning to the castle with his servant Courtney, Thadeous is given an ultimatum: join Fabious on his quest to rescue Belladonna or be banished from Mourne.
Visiting the Great Wize Wizard, Thadeous and Fabious learn that Leezar is attempting to fulfill a prophecy of a warlock having sex with a maiden when the two moons converge, impregnating her with a dragon that will allow him to take over King Tallious' kingdom. To destroy Leezar, they are given a magic compass that will lead them to the Blade of Unicorn, located within a labyrinth. On the way there, they learn Fabious's eunuch slave, Julie, has been reporting to Leezar of their progress and that the Knights Elite are serving the warlock. Fabious sends his mechanical bird Simon to tell the king of the betrayal by the Knights Elite and request reinforcements. Thadeous, Fabious and Courtney are captured by nymphs under their leader, Marteetee, who imprisons them at an arena, where Fabious kills off Marteetee's finest warrior. In retaliation, Marteetee summons a hydra-like monster to kill them. The brothers are rescued by Isabel, a warrior seeking revenge for her father's murder at Marteetee's hands.
Later that night, Thadeous learns that Isabel is also after Leezar for the slaughter of her brothers. The next day, the party learns too late that Isabel stole the compass from Thadeous. Fabious, finally angered by his brother's selfishness, decides to find the Blade of Unicorn alone as Thadeous and Courtney go to a tavern, where they find Isabel and retrieve the compass. But finding that his brother has been captured by Leezar's men, Thadeous wins Isabel over as they join forces, entering the labyrinth where they encounter a Minotaur. After becoming separated from the others, Thadeous retrieves the Blade of Unicorn and slays the Minotaur. Thadeous and his group make their way to Leezar's castle and free Fabious, giving him the Sword of Unicorn. As the others kill off Julie, Boremont and his men and Leezar's three witches, Fabious impales Leezar with the Blade of Unicorn, to prevent him from raping Belladonna.
After their victory, Isabel leaves for another quest and the heroes return home. Fabious and Belladonna marry, while Thadeous is approached by Isabel, who reveals that she has fallen in love with him. However, for them to have sex, he must first slay the witch who has cast a spell on her, locking her in a chastity belt. Though not in the mood to go out, Isabel's suggestion convinces him to go on a new adventure.
- Danny McBride as Prince Thadeous
- James Franco as Prince Fabious
- Natalie Portman as Isabel
- Zooey Deschanel as Belladonna
To be added