Prince Hector (Eric Bana) and his young brother Paris (Orlando Bloom) negotiate peace between Troy and Sparta. Paris has fallen in love with Helen (Diane Kruger), the wife of king Menelaus (Brendan Gleeson), and smuggles her to Troy with him. Infuriated, Menelaus vows revenge. Menelaus approaches his brother Agamemnon (Brian Cox), a king who has conquered every army of Greece, and now commands them. Agamemnon, who has wanted to conquer Troy for years (which would give him control of the Aegean Sea), uses this as a justification to invade Troy. General Nestor (John Shrapnel) asks him to take the legendary warrior Achilles (Brad Pitt), to rally the troops to the cause.
Odysseus (Sean Bean), a king of Mycenae commanded by Agamemnon, visits Phtia to persuade Achilles to fight, and finds him training with Patroclus (Garrett Hedlund), his cousin. Achilles, pondering his decision, visits his mother Thetis (Julie Christie) for advice. She tells him that, before he was even born, she knew this day would come. She also tells him that if he does not go to Troy, he will live a long, happy life and have children, but after he dies, his name will be forgotten and nobody will remember him. If he does go to Troy, he will find great glory in battle, his name will be written into history forever, but he will die there. Achilles, wanting his name to be remembered, chooses to go.
The Greeks sail for Troy. Achilles and the Myrmidons are the fastest rowers and land before anyone else. They kill many Trojans and desecrate the temple of Apollo. Briseis (Rose Byrne), a member of the Trojan royal family, is captured and taken as a prize to the Greeks, despite Achilles claiming her as a war prize himself.
Achilles and his Myrmidons do not fight the next day because of Agamemnon's unfair claim to Briseis. With Greeks surrounding Troy, Paris challenges Menelaus to a duel to settle things, using the Sword of Troy, his father claiming that the city's people have a future so long as