In ancient Greece, King Leonidas of Sparta is approached by a Persian messenger, who demands the submission of Sparta to King Xerxes' rule. Outraged by the demand and the messenger's arrogance, Leonidas kicks the envoy into a well, sparking a declaration of war with the mighty Persian Empire. Constrained by Spartan law and politics which forbid him to mobilize the full Spartan army, Leonidas defies the Ephors—oracle-priests—and selects a small force of 300 elite Spartan warriors to accompany him to the Battle of Thermopylae. These Spartans are determined to halt the advance of the Persian juggernaut and defend the freedom of Greece. Leonidas' fiercely loyal wife, Queen Gorgo, supports his decision and bids him farewell, hoping he can uphold the glory and valor of Sparta.

Before the massive conflict, Leonidas meets allies in Arcadia, who are impressed by the Spartans' prowess and resolve, though they are far outnumbered by the Persians. The Spartan king reveals his strategy to utilize the narrow pass of Thermopylae ("The Hot Gates") to neutralize the numerical advantage of the Persian forces. Leonidas and his men fortify the pass and prepare for a gruesome, unyielding confrontation against waves of Persian soldiers, including slaves, elite fighters, and grotesque creatures. The Spartans display extraordinary skill and bravery in the face of immense opposition, embodying the warrior ethos of their people and offering a powerful symbol of resistance against tyranny and overwhelming odds.