Thadeous and Fabious, portrayed by Danny McBride and James Franco respectively, are two royal brothers under the reign of King Tallious (Charles Dance). While Fabious stands out as a dashing and accomplished warrior, Thadeous grapples with a deep-seated inferiority complex and struggles to prove himself in quests, often opting for laziness over valor. Their lives take a dramatic turn when Fabious triumphantly returns from a battle against the notorious sorcerer, Leezar (Justin Theroux), and introduces the beautiful virgin Belladonna, played by Zooey Deschanel, whom he has rescued and intends to marry.
Despite being appointed the best man, Thadeous feels overshadowed and decides to skip the wedding after overhearing the negative chatter among Fabious's Elite Knights, particularly their leader Boremont (Damian Lewis). The joyous occasion soon turns chaotic when Leezar disrupts the ceremony, revealing his sinister plot and connection to Belladonna, a revelation that compels Thadeous to join forces with Fabious on a perilous adventure to save her.
Their journey begins with a visit to the eccentric Great Wise Wizard, who unveils Leezar's nefarious plan to fulfill a prophecy involving an unholy union with a maiden during a celestial event, aiming to unleash a dragon to bolster his quest for dominion over King Tallious's kingdom. Armed with a magical compass pointing toward the legendary Sword of Unicorn hidden deep within a labyrinth, the brothers embark on a risky expedition.
As they navigate through dangers—including betrayal from Fabious's supposed ally, Julie (Toby Jones), and confrontations with Leezar's Elite Knights—they stumble upon a band of nymphs led by the cunning Marteetee (John Fricker). After a brutal battle in an arena, where Fabious emerges victorious against Marteetee's finest warrior, the stakes rise with the summoning of a hydra-like monster.
The plot thickens as they are unexpectedly saved by Isabel (Natalie Portman), a fierce warrior seeking revenge against Marteetee for her father's death. As they share camp, Thadeous inadvertently reveals their mission and the compass's location to Isabel. Tensions mount when she steals the compass, leaving Thadeous and Fabious on the brink of despair. However, realizing his brother has fallen victim to Leezar’s men, Thadeous manages to rally forces with Isabel to infiltrate the labyrinth once more, facing a minotaur along the way.
A transformation occurs as Thadeous bravely retrieves the Sword of Unicorn and confronts the beast, emerging victorious and sporting the minotaur's severed trophy as a badge of honor. With the group reassembled, they advance to Leezar's fortress, culminating in an epic showdown. Fabious, wielding the Sword of Unicorn, ultimately vanquishes Leezar just in time to save Belladonna, restoring peace to the kingdom.
In the aftermath, while Fabious ties the knot with Belladonna, Thadeous finds himself retreating to his quarters. Just as he's about to end his evening alone, Isabel surprises him with her admission of love and a proposal for a new adventure—only this time, involving the challenge of slaying the witch responsible for her enchanted chastity belt. Though initially reluctant, Thadeous finds himself tempted by Isabel's charm, ready to embark on yet another quest.
