Through Aristodemus' narration, the life of young Leonidas is depicted, chronicling his journey from a boy to a man through the Spartan doctrine of Agoge. The climax of this is when the young Leonidas kills a giant wolf using an improvised spear, after which he returns to Sparta and is crowned king. Years later, Persian messengers arrive at the gates of Sparta demanding its submission to King Xerxes. Offended by their threats and behavior, Leonidas and his guards kick the messengers down a well. Knowing that these actions will precipitate a Persian attack, Leonidas visits the Ephors, ancient priests whose blessing he needs to convince the Spartan council to authorize going to war. He proposes a strategy to repel the numerically superior Persians by using the terrain of Thermopylae (the Hot Gates); his plan involves funneling 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. After Leonidas departs a messenger from Xerxes appears, rewarding the Ephors for their covert support and revealing that they have been corrupted by Xerxes.


Denied by the Ephors, Leonidas follows his plan anyway, deciding to set out with only 300 soldiers. While he does not require the council's permission for this, taking such a small force turns what had been a bold strategy into a certain suicide mission. Leonidas hopes that the sacrifice of himself and his men will spur the council to defy the Ephors and all of Greece to unite against the threat to freedom and democracy (represented by Greece) posed by slavery and tyranny (represented by Persia).


Along the way to Thermopylae, the Spartans are joined by Arcadians and various other Greeks. Along the path, the band encounters a sacked town burning on fire, apparently caused by Persian Immortals. Only a young child is left to live. They construct a wall at Thermopylae to contain the approaching Persian advance. Meanwhile, Leonidas encounters Ephialtes, a hunchbacked Spartan whose parents fled Sparta to spare him certain infanticide. Ephialtes asks to redeem his father's name by joining Leonidas, warning him of a secret path the Persians could use to outflank and surround them. Leonidas is sympathetic to the eager warrior but rejects him, as Ephialtes cannot properly hold a shield, which would compromise the Spartans' phalanx formation.