At Thermopylae, the allied Greek nations deployed a small force of four thousand Greek heavy infantry against the invading Persian army of two million strong. Leading the Greeks was a small force of three hundred Spartans, chosen because they were all "sires"— men who had to have sons who could preserve their blood line, should they fall in battle.
Thermopylae was the only gateway into Greece for the Persian army, and presented the perfect choke point— a narrow pass bordered by a huge mountain wall on one side and a cliff drop-off to the sea on the other. This area decreased the Persians' advantage of having large numbers. Delaying the Persian advance here would give the Greek allies enough time to ready a larger, main force to defend againstthe Persians. The battle takes place simultaneously with the sea battle at Artemisium, where the Allied Greek forces hoped to protect the flank of the army at Thermopylae whilst not being cut off themselves. The Greeks were at a disadvantage at Artemisium, as at Thermopylae - the Persians outnumbered the Allies, and most of the Athenian ships were newly built and manned by inexperienced crews - and both sides suffered heavy losses in the sea battle.
The novel is told from either the perspective of the royal scribe to the Persian king Xerxes, as he records the story of Xeones, after the battle, or in the first person from Xeones' point of view. Though Xeones is critically wounded in the battle, the Persian King Xerxes orders his surgeons to make every effort to keep the captive squire alive. Much of the narrative explores Spartan society, particularly the agoge, which is the military training program which all young Spartan boys must complete to become citizens of Sparta. The novel also details the heroics of several dozen Spartans, including the King of Sparta, Leonidas, the Olympic champion Polynikes, a young Spartan warrior named Alexandros, and the Spartan officer Dienekes. Pressfield employs detailed descriptions of the Spartan phalanx in battle, as well as the superior training and discipline of the Spartan warriors.