A battle between the Greek armies of King Agamemnon of Mycenae and King Triopas of Thessaly is quickly averted when the great warrior Achilles, fighting for Agamemnon, defeats Boagrius, Triopas' champion, in single combat after Achilles was initially absent from the battle. Thessaly joins Agamemnon's loose alliance comprising all of the Greek kingdoms.
Prince Hector of Troy and his younger brother Paris negotiate a peace treaty with Menelaus, King of Sparta. However, Paris is having an affair with Menelaus' wife, Queen Helen, and smuggles her aboard their home-bound vessel, much to Hector's dismay. Upon learning of this, Menelaus meets with Agamemnon, his elder brother, and asks him to help take Troy. Agamemnon agrees, as conquering Troy will give him control of the Aegean Sea. Agamemnon has Odysseus, King of Ithaca, persuade Achilles to join them. Achilles, who strongly dislikes Agamemnon, eventually decides to go after his mother Thetis tells him that though he will die, he will be forever glorified.
In Troy, King Priam is dismayed when Hector and Paris introduce Helen, but welcomes her and decides to prepare for war. The Greeks eventually invade and take the Trojan beach, thanks largely to Achilles and his Myrmidons. Achilles has the temple of Apollo sacked and has a brief fight with Hector and his army. They claim Briseis — a priestess and the cousin of Paris and Hector — as a prisoner afterward. He's angered when Agamemnon spitefully takes her from him and decides that he won't aid Agamemnon in the siege.
The Trojan and Greek armies meet outside the walls of Troy; during a parley, Paris offers to duel Menelaus personally for Helen's hand in exchange for the city being spared. Agamemnon, intending to take the city regardless of the outcome, accepts. Menelaus wounds Paris, causing him to cower at the foot of Hector. When Menelaus attempts to kill Paris despite his victory, he himself is killed by Hector. In the ensuing battle, Hector kills Ajax after a brief duel, and many Greek soldiers fall to the Trojan defenses with Achilles and the Myrmidons watch from a distance during the battle. On Odysseus' insistence, Agamemnon gives the order to fall back. After Ajax and Menelaus are cremated in the Greek camp, Agamemnon and Odysseus argue about why they lost the battle. Agamemnon gives Briseis to the Greek soldiers for their amusement, but Achilles saves her from them. Later that night, Briseis sneaks into Achilles' quarters to kill him; instead, she falls for him and they become lovers. Achilles then resolves to leave Troy, much to the dismay of Patroclus, his cousin and protégé.
Despite Hector's objections, Priam orders him to retake the Trojan beach by daybreak and force the Greeks home; the attack unifies the Greeks and the Myrmidons enter the battle. Hector duels a man that he believes to be Achilles and kills him, only to discover that it was actually his cousin Patroclus. Distraught, both armies agree to stop fighting for the day. Achilles is informed of his cousin's death by Eudorus and vows revenge after striking him. Wary that Achilles will surely seek vengeance, Hector shows his wife Andromache a secret tunnel beneath Troy; should he die and the city fall, he instructs her to take their child and any survivors out of the city to Mount Ida.
The next day, Achilles arrives outside Troy and challenges Hector. Knowing death would await him, Hector says his goodbyes to his loved ones, including his wife and son. The two duel outside the gates and initially appear evenly matched, but Hector is slowly worn down until Achilles lands the killing blow. Achilles then drags his corpse back to the Trojan beach. Priam, in disguise, sneaks into the camp and implores Achilles to return Hector's body for a proper funeral. Ashamed of his actions, Achilles agrees and also states that Hector was the best that he had ever fought. He allows Briseis to return to Troy with Priam, promising a 12-day truce so that Hector's funeral rites may be held in peace. He also orders his men to return home without him after apologizing to Eudorus for striking him.
Agamemnon declares that he will take Troy regardless of the cost. Concerned, Odysseus concocts a plan to infiltrate the city. After seeing a carving of a horse by a Greek soldier, he has the Greeks build a gigantic wooden horse as a peace offering and abandon the Trojan beach, hiding their ships in a nearby cove. Despite objections from Paris, who requests for it to be burned down, Priam orders the horse to be brought into the city after Archeptolemus views it as a gift intended for calming the gods. A Trojan scout later finds the Greek ships hiding in the cove, but he is quickly shot as he is about to alert the city. That night, Greeks hiding inside the horse emerge, attacking the sleeping Trojans, and open the city gates for the Greek army, commencing the Sack of Troy. While Andromache and Helen guide the Trojans to safety through the tunnel, Paris gives the Sword of Troy to a young boy named Aeneas, instructing him to protect the Trojans and find them a new home. Agamemnon kills Priam and captures Briseis, who then kills Agamemnon using a concealed knife in her hand. Achilles fights his way through the city and reunites with Briseis after killing 2 Greek soldiers. Paris, seeking to avenge his brother, shoots an arrow through Achilles' heel and then several into his body. Achilles extracts all the arrows but the one in his heel. He then bids farewell to Briseis and watches her flee with Paris before dying.
In the aftermath, Troy is finally taken by the Greeks and a funeral is held for Achilles. Odysseus personally cremates his body, while the surviving Trojans flee to Mount Ida.
Brad Pitt as Achilles, Greece's greatest warrior and leader of the Myrmidons
Eric Bana as Hector, Crown Prince of Troy, commander of the Trojan armies and Paris' older brother
Orlando Bloom as Paris, Prince of Troy and Hector's younger brother
Diane Kruger as Helen, former Queen of Sparta who became a Princess of Troy
Brian Cox as Agamemnon, King of the united Greek city-states
Sean Bean as Odysseus, King of Ithaca
Brendan Gleeson as Menelaus, King of Sparta and younger brother of Agamemnon
Rose Byrne as Briseis, priestess of the Trojan Temple of Apollo
Saffron Burrows as Andromache, Hector's wife and Crown Princess of Troy
Julie Christie as Thetis, Achilles' mother
Peter O'Toole as Priam, King of Troy
Garrett Hedlund as Patroclus, Achilles' younger cousin
John Shrapnel as Nestor, Agamemnon's advisor
Nigel Terry as Archeptolemus, the Trojan high priest
James Cosmo as Glaucus, second-in command of the Trojan armies
Julian Glover as Triopas, King of Thessaly
Vincent Regan as Eudorus, Achilles' right-hand man of the Myrmidons
Trevor Eve as Velior
Tyler Mane as Ajax
Nathan Jones as Boagrius, Triopas' champion