In 480 BC, the ambitious, cruel and merciless King Xerxes (David Farrar) of Persia invades Greece with his huge army to extend his vast slave empire. The brave Spartan army is the great hope to free and unite Greece, and king Leonidas (Richard Egan) promises to the council of the Greek Stats to defend the passage of Thermopylae, the only way by land to reach Athens. However, he is betrayed by the politicians of Sparta and stays alone with his personal body guard army composed of three hundred warriors only. Using courage and great knowledge of strategies of war, he defends Thermopylae until a treacherous goatherd tells King Xerxes a secret goat passage leading to the back of Leonidas's army.