It's the 12th century BC and King Agamemnon (BRIAN COX) of Mycenae and his army are in Thessaly, Greece, looking to expand their military might and empire. Rather than suffer great losses, he and an opposing King agree to let their greatest warriors battle. Agamemnon then calls upon the legendary Achilles (BRAD PITT) who easily dispatches his much larger opponent. Only looking to make a permanent name for himself for all eternity, Achilles barely has any allegiance to Agamemnon and is a constant thorn in his side.
He must call upon him again, however, when his brother Menelaus (BRENDAN GLEESON), wants revenge for Trojan Prince Paris (ORLANDO BLOOM) running off with his wife, Helen of Sparta (DIANE KRUGER). Paris' warrior brother, Hector (ERIC BANA), isn't pleased to learn of this development and once back in Troy, urges their father, King Priam (PETER O'TOOLE), to return the young woman.
Priam, however, puts too much faith in his high priests and their interpretation of how the gods will react. Accordingly, and with Agamemnon realizing that a victory over Troy would give him complete control over the Aegean, the Greeks amass the largest naval force known to man and set sail for the foreign land. With them is Achilles and his younger and inexperienced cousin, Patroclus (GARRETT HEDLUND), who've been convinced by King Odysseus (SEAN BEAN) and Achilles' own mother, Thetis (JULIE CHRISTIE), to join the quest.
With the Greek forces landing on Trojan soil and taking Hector's cousin Briseis (ROSE BYRNE) into custody, and Hector rallying his troops for battle, the stage is then set for the great Trojan War.