King Agamemnon (Brian Cox) of Mycenae is in Thessaly, Greece, with his army looking to expand territory and influence. On the battlefield, Agamemnon's soldiers prepare to engage in combat against the army under the Thessalonian king, Triopas (Julian Glover). Rather than suffer great losses, Triopas agrees to Agamemnon's proposal to settle the matter in the traditional way - through a decisive match between the best warriors of the opposing armies. Achilles (Brad Pitt) is summoned by Agamemnon, and after arriving, easily kills the Thessalonian champion Boagrius (Nathan Jones). Accepting defeat, Triopas presents Achilles with a scepter as a token for his king. But Achilles refuses, saying Agamemnon is not his king.
In Sparta, Prince Hector (Eric Bana) and his young brother Paris (Orlando Bloom) negotiate an end to the war between the outlying kingdom of Troy and Sparta. On the last day of a week-long peace festival, Paris manages to smuggle Helen (Diane Kruger), Menelaus' (Brendan Gleeson) wife, back to Troy with him. Infuriated by Helen's disappearance, Menelaus vows revenge. Meanwhile, Agamemnon (Menelaus' brother), who had for years harbored plans for conquering Troy (an achievement which would give him complete control of the Aegean Sea), decides to use his brother's situation as a justification to invade Troy. He is advised by his general, Nestor (John Shrapnel), to call upon Achilles to fight for the Greeks, ensuring they can rally enough troops to the cause.
Odysseus (Sean Bean) is then sent to Phtia to convince Achilles and his Myrmidons to fight, and finds him training with Patroclus (Garrett Hedlund), his cousin and student. Odysseus says the fleet will be sailing to Troy in three days, and that this war will never be forgotten. Achilles consults his mother, Thetis (Julie Christie), and she tells him that should he stay, he will find peace and a long life, but his name will be forgotten. Should he go, he will find everlasting glory, but will die in combat. Achilles decides to go to war.
The Greeks land at Troy and take control of the beach on the first day of the war. Achilles and the Myrmidons kill many Trojans and also desecrate the seaside temple of Apollo, slaying the unarmed priests that reside there. Within the temple, Achilles and Hector meet but do not fight, with Hector outnumbered but allowed to leave. Briseis (Rose Byrne), a member of the Trojan royal family who has chosen to dedicate her life to service to the gods, is captured and taken as a prize to Achilles. However, he treats her with kindness, which makes her initially cautious.