The film opens with a Spartan elder inspecting a talking baby ogre. The baby vomits on the inspector and is then discarded with a punt off the hill. Next, he inspects a Vietnamese baby, and Brangelina instantly adopts it. Baby Leonidas is then inspected, having a six-pack, biceps, and beard from birth. He is accepted as a Spartan and prepared for kinghood through his childhood training, from fighting his grandmother to enduring torture. Leonidas (Sean Maguire) is then cast out into the wild, and survives the harsh winter while killing a giant dancing penguin. Returning a king for his inauguration ceremony, Leonidas sees Margo (Carmen Electra) dancing and asks her to marry him, to which she responds by giving him the combination to her chastity belt.
Years later, Leonidas is training when Captain (Kevin Sorbo) informs him that a Persian messenger has arrived. The messenger has come to present Xerxes' demands for Sparta's submission. Leonidas arrives to greet the messenger in the Spartan way (high-fives for the women and open mouth tongue kisses for the men). After growing angry with both the messenger's disrespect and making out with his wife, Leonidas kicks him, the messenger's bodyguards, and then several other people he simply disliked, ranging from Britney Spears (Nicole Parker), Ryan Seacrest, Michael Jackson and Kevin Federline (Nick Steele) to Sanjaya Malakar (Tony Yalda) and the American Idol judges into "the pit of death". As Leonidas walks off he turns to a column that has a switch that reads "Garbage Disposal", and flips the switch causing the celebrities to spiral to their "death".
Resolving to face the Persians, Leonidas visits the Oracle, proposing an "erotic-sounding" strategy to repel the numerically superior enemy after offering the priests various skin-care lotions for guidance. The Oracle, Ugly Betty (Crista Flanagan), reveals that Leonidas will die should he go to war. The next day, Leonidas meets the soldiers assembled for his departure to Thermopylae, and finds that only 13 (not 300) were accepted in the army, since there were stringent specifications to be accepted "Hunky with deep Mediterranean tans, and well-endowed". Three among them include Captain, his son, Sonio, and a slightly unfit Spartan named Dilio, who, as the Captain states, "Has a lot of heart...and nice man boobs." Once at the Hot Gates, they encounter Paris Hilton (also played by Parker), who tells Leonidas and the Captain about a secret goat path above the Hot Gates that Xerxes could use to outflank the Spartans. When she asks to be made a Spartan soldier Leonidas rejects her as unqualified.
Leonidas and his platoon soon face off with Xerxes' messenger and his Immortals, beating them in a dance contest before driving them off a cliff. Xerxes (Ken Davitian), impressed, personally approaches Leonidas and attempts to bribe him in a Deal or No Deal fashion. Despite the soldiers' encouragements, the Spartan king declines, saying that he will instead make the "God King" fall. The Spartans then face the Persian army in a "Yo Momma" fight, ending with a victory, in spite of Dilio having his eyes scratched out. Though victory seemed to be in the Spartans' grasp, Paris Hilton betrays the Spartans and reveals the location of the goat path to Xerxes, having been promised having her hump removed as one of her traitorous rewards. Using a CGI army, Xerxes meets the 12 remaining Spartans and the war is on.
Meanwhile, back in Sparta, Queen Margo sexually submits to Traitoro in order to persuade him to send more troops to assist Leonidas. He states that he will finally lose his virginity while using his cell phone to take pictures. However, he reveals publicly that she has not been chaste. The anger at this revelation provokes a symbiote suit to envelop her, mirroring the parasitic symbiote Spider-Man suit in Spider-Man 3. She fights with Traitoro, who, in line with the parody, becomes the Spider-Man 3 villain the Sandman. Margo wins the battle with a vacuum cleaner - and Traitoro's treachery is revealed when it is discovered that Xerxes is in Traitoro's Top Five, whereupon Sparta agrees to send more troops.
At the Battle of Thermopylae, the Persians introduce their secret weapons: Ghost Rider, who is quickly defeated, and Rocky Balboa, who decapitates Sonio with one right hook but is killed by a vengeful Captain with a Botox-laced syringe. Captain then gets killed by a spear, thrown through his torso by Xerxes.
Captain's death enrages Leonidas, and the battle sequence switches to a Grand Theft Auto-esque format as he runs through the Persian Army and corners Xerxes. While running away, Xerxes finds the "Transformer Cube" and harnesses its power to combine with a convertible to become "Xerxestron", playing "Leave Britney Alone" videos to prove his "godly power". But Xerxestron ends up delivering on Leonidas's promise to "fall", as his power cord disconnects and he falls forward, crushing the surviving Spartans under his bulk. Thus the narrator concludes, "Leonidas was true to his word: a god-king did fall... but unfortunately, right on them!" The blind Dilio, who left prior to the final battle, eventually returns to Sparta to tell of Leonidas' final moments. A year later, Dilio leads a force of one hundred real Spartans and several thousand CGI-ones to defeat the Persians, but the blind warrior ends up going the wrong way and leads the Spartans to Malibu. They end up crashing into Lindsay Lohan as she once more leaves drug rehab, causing her to expose her bare groin and fly off screen.
The film ends with a musical number set to Gloria Gaynor's "I Will Survive" performed on American Idol by all of the characters in the film. It also spoofs Britney Spears' VMA comeback performance.