Thermopylae : one of the most famous battles of antiquity. For three days, a few thousand Greeks, commanded by the Spartan king Leonidas, blocked the 2 million-strong army of the Persian king Xerxes in a narrow mountainous gorge, along the Aegean Sea, in northern Greece. A mythical battle, which will end with the sacrifice of Leonidas and 300 brave Spartans who accompanied him; a sacrifice that, by delaying the advance of the Persians, will save the Greek civilization. From the story of the battle that Herodotus has left us, the film recounts this tragic and glorious episode, which it examines with the expertise of contemporary historians, and retraces step by step the course of the battle, reconstructed in animation, thanks to modern 3D techniques.