Thadeous and Fabious (Danny McBride and James Franco) are two sons of King Tallious (Charles Dance). They are both warriors, but Fabious is dashing and skilled and Thadeous is lazy and ineffectual with both an inferiority complex and poor track record in quest taking. While celebrating his latest victory over the evil sorcerer who has been ravaging Tallious's kingdom, Leezar (Justin Theroux), Fabious introduces the virgin Belladonna (Zooey Deschanel) he freed from a tower and wishes to marry her. Though he is made the best man, Thadeous skips the wedding after overhearing Fabious's Elite Knights, led by Boremont (Damian Lewis), talk about him negatively. But the wedding is then crashed by Leezar, revealing himself to be the one who placed Belladona in the tower before spiriting her away. Returning to the castle with his servant Courtney (Rasmus Hardiker), Thadeous is forced into joining Fabious on his quest to rescue Belladonna.
Visiting the perverted Great Wise Wizard, Thadeous and Fabious learn that Leezar is attempting to fulfill a prophecy of a warlock having hard-core 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 would lead them to the fabled Sword of Unicorn which is located within a labyrinth. On the way there, after finding that Fabious's slave, Julie (Toby Jones) has been reporting to Leezar of their progress, Thadeous and Fabious learn that Elite Knights are also serving the warlock and escape from them alongside Courtney. While collecting themselves at a river, after his brother sends his mechanical bird Simon to tell the king of the Elite Knights' betrayal and request reinforcements, Thadeous, Fabious and Courtney are captured by nymphs under their leader, Marteetee (John Fricker), who imprisons them at an arena where Fabious kills off Marateetee's finest warrior. In retaliation, Marteetee summons his hydra-like familiar to kill them.
However, Thadeous and Fabious are rescued by Isabel (Natalie Portman), a warrior that is seeking revenge for her father's murder at Marteetee's hands. Later that night, as Fabious and Courtney leave them for the mood to set in, Thadeous learns that Isabel is also after Leezar for the slaughter of her brothers before accidentally revealing their quest and the compass to her. The next day, the party learn too late that Isabel stole the compass from Thadeous and ran off. Finally infuriated of his brother's selfish behavior as they arrive to a village, Fabious decides to find the Sword of Unicorn alone as Thadeous and Courtney go to a tavern, where they find Isabel and steal the compass back. 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. Getting separated from the others, Thadeous retrieves the Sword of Unicorn and, after a test of worth, slays the minotaur as Isabel uses a panflute to soothe the monster as he is about to rape Courtney. A changed man, proudly wearing the minotaur's severed penis as a trophy necklace when unable to get one of the beast's horns, Thadeous and his group make their way to Leezar's castle and free Fabious while giving him the Sword of Unicorn. As the others kill off Julie and Boremont's men along with Leezar's mothers, Fabious uses the Sword of Unicorn to end Leezar's life before he is able to rape Belladonna, saving the kingdom.
After their victory, the heroes go back home, but Isabel goes on another quest. Fabious and Belladonna marry as Thadeous retreats to his bedroom to masturbate before going to bed. There, he 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 that cast a spell on her, locking her in a chastity belt. Though he is not in the mood to go out, Isabel's suggestion to cuddle convinces him to go on a new adventure.