Starring : Gerard Butler, Lena Headey
Dilios, a Spartan soldier, narrates the story of Leonidas, from boyhood to the throne of Sparta. Years later, a Persian messenger arrives at the gates of Sparta demanding it submit to King Xerxes. In response to this demand, Leonidas and his guards kick the messenger into a pit. Knowing this will prompt a Persian attack, Leonidas visits the Ephors—ancient, leprosy-ridden priests whose blessing he needs before the Spartan council will authorize going to war. He proposes they repel the numerically superior Persians by using the terrain of Thermopylae (the Hot Gates), and funnel the Persians into a narrow pass between the rocks and the sea. The Ephors consult the Oracle Pythia, who decrees that Sparta must not go to war during their religious festival. As Leonidas departs two agents of Xerxes appear (one of them - Theron - a Spartan) who bribe the Ephors with concubines and money.
Leonidas follows his plan anyway, setting out with only 300 soldiers, whom he calls his personal guard to avoid needing the council's permission. Though he knows it is a certain suicide mission, he hopes the sacrifice will spur the council to unite against Persia. Along the way to Thermopylae, the Spartans are joined by Arcadians and other Greeks. At Thermopylae, they construct a wall to contain the approaching Persian advance. As construction goes on, Leonidas meets Ephialtes of Trachis, a hunchbacked Spartan in exile whose parents fled Sparta to spare him certain infanticide. Wanting to redeem his father's name, he asks to join the fight, and warns Leonidas of a secret path the Persians could use to outflank and surround them. Though Leonidas is sympathetic with Ephialtes' will to fight, he turns him down, as Ephialtes cannot properly hold a shield, which would compromise the Spartans' phalanx formation.
Before the battle, the Persians demand that the Spartans lay down their weapons. Leonidas refuses, and with their tightly-knit phalanx formation the Spartans use the narrow terrain to repeatedly rebuff the advancing Persian army. Xerxes personally parleys with Leonidas, offering him wealth and power in exchange for his loyalty and surrender. Leonidas declines and Xerxes sends his elite guard, the feared Immortals, to attack them, but the Spartans successfully dispatch them although they suffer a few casualties of their own. Xerxes then sends a number of exotic weapons at the Spartans, including black powder bombs and giant war beasts, but all of these attacks fail. Angered by Leonidas' rejection, Ephialtes defects to the Persians and informs them of the secret path. When they realize Ephialtes' treachery, the Arcadians retreat and Leonidas orders Dilios to return to Sparta to tell the Council of their sacrifice. Though Dilios had recently lost his left eye in combat, he is still fit for battle, but Leonidas decides to use Dilios' gift for storytelling to appeal to the Spartan council. Though reluctant to leave his brothers behind, Dilios leaves with the Arcadians.
In Sparta, Theron rapes Gorgo, Queen of Sparta (Leonidas' wife), in return promising his help in persuading the Spartan council to send reinforcements to Leonidas. Following her address to the Council, Theron publicly betrays the Queen, prompting the councilmen to cry out in outrage and Gorgo to kill him in a fit of anger. As she kills him, she repeats what he told her just prior to raping her: "This will not be over quickly, you will not enjoy this". The dagger pierces his purse, spilling Persian coins from his robe, revealing his role as traitor, and the Council agrees to unite against Persia.
At Thermopylae, the Persians use the goat path to surround the Spartans. Xerxes' general demands their surrender, again offering Leonidas titles and prestige. Leonidas seemingly bows in submission, allowing one of his men to leap over him and kill the general instead. Furious, Xerxes orders his troops to attack. Leonidas rises and hurls his spear at Xerxes, cutting the King on the cheek, thus making good on an earlier promise to make "the 'god'-King bleed." Visibly disturbed by this reminder of his own mortality, Xerxes watches as all the Spartans are slaughtered by a massive barrage of arrows. Moments before his death, Leonidas pledges his undying love to Gorgo.
Concluding his tale before an audience of Spartans on the edge of the battlefield a year after Thermopylae, Dilios relates how the Persian army is depleted by desertions, out of fear, and the heavy casualties they suffered at the hands of a mere 300 Spartans. Word of the valiant resistance of the 300 Spartans spread across Greece, inspiring the different city-states to unite against the Persians. Now the Persians face 10,000 Spartans leading 30,000 free Greeks. Although still outnumbered 3 to 1, Dilios declares that the Greeks shall be victorious, and praises the sacrifice of King Leonidas of Sparta. He then leads the Greeks in a charge against the Persian army, beginning the Battle of Plataea.