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, easily 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 peace 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 destroy Troy and retrieve his wife. 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 even 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 confrontation with Hector. He claims Briseis, a priestess and the cousin of Paris and Hector, as a prisoner afterward. However, Agamemnon spitefully takes Briseis from Achilles, and Achilles decides he will not further aid Agamemnon in the siege.

The next day, 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 is killed by Hector. Agamemnon then leads an attack on the Trojan army which is outnumbered two to one. The armies collide violently with many falling on both sides. Hector spots and kills Ajax after a brief but fierce duel, and many Greek soldiers fall to the Trojan archers and infantrymen with Achilles and the Myrmidons watching the battle from a distance. 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 to return home. The Trojans attack at night, shooting fire arrows in front of the Greek camp while rolling large hay balls towards the camp. The attack wakes the Greeks who assemble before the camp. Achilles rallies the Greeks and the Myrmidons charge the Trojan army along with the rest of the Greeks. Hector cuts down many Greeks and duels a man that he believes to be Achilles and kills him, only to discover that it was actually 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 falls, he instructs her to take their child and any survivors out of the city to Mount Ida.