Plot
Queen Gorgo of Sparta narrates about the Battle of Marathon, in which King Darius of Persia was killed by Themistocles of Athens. Darius's son, Xerxes, witnesses his father's death and is advised to never wage war against the Greeks. Darius's naval commander, Artemisia, persuades Xerxes to become a god and sends him on a journey through the desert. Xerxes reaches a cave and bathes in an otherworldly liquid, emerging as a "God-King". He returns to Persia and declares war on Greece to avenge his father.
As Xerxes' forces advance towards Thermopylae, Themistocles meets with the council and convinces them to provide him with a fleet to engage the Persians at sea. Themistocles travels to Sparta to ask King Leonidas for help but is informed by Dilios that Leonidas is consulting the Oracle, and Gorgo is reluctant to side with Athens. Themistocles reunites with his old friend Scyllias, who infiltrated the Persian troops, and reveals Artemisia was born Greek but defected to Persia after her family was murdered by Greek hoplites. A Persian emissary took her in and trained her, and she eventually rose to become a naval commander. Themistocles also learns that Leonidas has marched to fight the Persians with only 300 men.
Themistocles leads his fleet of fifty warships and several thousand men, which include Scyllias, Scyllias's son Calisto, and Themistocles' right-hand man Aeschylus to the Aegean Sea, starting the Battle of Artemisium. They ram their ships into the Persian ships and attack them before retreating. The following day, the Greeks feign a retreat and lead the Persian ships into a crevice, where they become stuck. The Greeks attack the Persian ships from the cliffs above. Impressed with Themistocles, Artemisia brings him onto her ship and attempts to seduce him to rally the Persian side as her second-in-command, but he refuses her offer.
The Persians attack the Greek ships with tar and flame bombs, but an Athenian kills one of the Persians, who falls into the tar carrying a torch, damaging ships from both sides. Themistocles is thrown into the sea by an explosive carried onboard by a Persian swimmer, nearly drowns before being rescued by Aeschylus, and is by Scyllias's side as he succumbs to his arrow injuries inflicted by Artemisia in echo of King Darius's death. Believing Themistocles to be dead, Artemisia and her forces withdraw. After recovering from his injuries, Themistocles learns that only a few hundred of his warriors and six of his ships survived the disastrous attack.
Daxos, an Arcadian general, tells Themistocles that Leonidas and his 300 men were killed after Ephialtes betrayed the Greeks to Xerxes. Themistocles returns to Athens and confronts Ephialtes, who reveals that Xerxes plans to attack and burn Athens. Ephialtes regrets his betrayal and welcomes death but is spared so he can warn Xerxes that the Greek forces are gathering at Salamis. Themistocles visits Gorgo in Sparta to ask for help, but Gorgo, mourning Leonidas's death, refuses. Before departing, Themistocles urges Gorgo to avenge Leonidas.
In Athens, Xerxes' army is laying waste when Ephialtes arrives to deliver Themistocles' message. Upon learning he is alive, Artemisia leaves to ready her navy for battle. Themistocles inspires all of his remaining forces to continue fighting. The remaining Greek ships charge into the Persian ships, beginning the decisive Battle of Salamis. Themistocles and Artemisia engage in a duel, which ends in a stalemate.
Gorgo arrives at the battle along with ships from numerous Greek city-states including Delphi, Thebes, Olympia, Arcadia, and Sparta, all united against the Persians. Xerxes, watching the battle from a cliff, turns back, acknowledging his naval defeat and continuing the march of his army. Artemisia attacks Themistocles, but he stabs and kills her. Accompanied by Dilios, Themistocles and Gorgo lead the entire Greek army against the Persians.
