In 480 B. C. Emperor Xerxes of Persia plans a massive invasion of Greece. Themistocles of Athens calls an emergency meeting of all the Greek rulers and King Leonidas of Sparta suggests that his legions check the Persians at the narrow pass of Thermopylae, thereby giving the other Grecian states time to mobilize. But Sparta's ultimate ruling body, the Council of Euphors, decrees that there can be no fighting during the religious festival in progress; and Leonidas is left with only his personal retinue of 300 men. As they make their way to Thermopylae, they are followed by a young Spartan soldier, Phylon, who has been falsely accused of being a traitor's son, and his beloved Ellas, the niece of Leonidas. The Persians attack, and great numbers of their soldiers are slaughtered in the narrow pass. But the Spartans are betrayed by a goatherd, Ephialtes, who tells the Persians of a little-known trail by which they can traverse the mountain and attack the Spartans from the rear. Though warned of the imminent danger, Leonidas chooses to make a stand. After sending Phylon and Ellas back to Athens to warn the Greeks and tell them of the Spartans' resistance, he and his men form a wedge of shields and hold their ground until they are totally overwhelmed by the Persians. Their sacrifice serves to unite the Greeks, and a few months later the Persian army is annihilated at the Battle of Plataea.