It is Achilles' mother, the sea nymph Thetis, who inadvertently causes him to change his mind. She tells him that his fate lies in his hands. He can fight and become a hero who will be remembered forever but die young, or he can live a long life, surrounded by his loved ones, and be consigned to oblivion. Achilles does not hesitate and goes off to join the Greek forces. Meanwhile, the seer Calchas reveals that, offended by Agamemnon, the goddess Artemis is preventing ships from leaving port. Agamemnon agrees to sacrifice his daughter Iphigenia so he can assemble the largest fleet ever in order to attack Troy.

But Paris loves Helen and will hear nothing of it. In the Greek camp, a mysterious plague has been raging for several days. Calchas the seer tells the assembled kings that Apollo sent it as revenge for Agamemnon enslaving the beautiful Chryseis, the daughter of a priest who complained to the god. Achilles demands that Agamemnon return the girl to her father. Agamemnon finally accepts but says that Achilles must give him his protégée, Briseis, as compensation. Seething with rage, Achilles swears that he will fight no more and goes into isolation. His mother, Thetis, goes to Zeus and asks him to defend Achilles. Remembering who he once loved, the mighty Olympian god decides to take the side of the Trojans and sends a dream to Agamemnon to set a trap for him.

But just as Menelaus is about to kill Paris, Aphrodite plucks him from the battlefield. Zeus is furious to see the gods interfering in this war, but he cannot prevent it. Athena returns to the battle and grants the young king Diomedes great fury and the ability to distinguish gods from men. He attacks the goddess Aphrodite, who is there to defend her son Aeneas, injuring her on the hand. Then he takes on Ares, the god of war, who is there to defend Aphrodite's honour. As the battle rages, Achilles sits indifferently playing the lyre in his tent.

Hector, the king of Troy's eldest son, is confident. His sister, Cassandra, is much less so. She has always known that Paris would bring misfortune to the Trojans, and she forces her mother, Hecuba, to admit it. The battle resumes, and when Poseidon gets involved, so too does Zeus, weighing the two sides' fates and favouring the Trojans. The Greeks know that victory is impossible for them without Achilles their great hero. Agamemnon agrees to send a delegation to ask him to return to combat. But will Achilles accept their appeals?

The wall built by the Greeks threatens to collapse. Angry about not being able to intervene, Hera concocts a plan and seduces her husband Zeus using Aphrodite's magic girdle. While he is sleeping, she rushes to warn Poseidon, who is also riled by having to obey orders from the god of Olympus. He rallies the Greek troops who retake the upper hand. Ajax takes on Hector, the eldest son of the king of Troy, and injures him. When Zeus wakes up, he is horrified to discover that Hector is dying.

Achilles refuses but agrees to let Patroclus wear his armour and lead the Myrmidons to save the Greeks. Believing they have seen Achilles, the Trojans beat a retreat. After killing Sarpedon, Zeus's son, Patroclus is filled with confidence and marches on Troy. First he is pushed back by Apollo, then he is killed by Hector, the eldest son of King Priam. Upon hearing this news, Achilles breaks down and vows to avenge the death of his friend. His mother Thetis promises to return the following morning with new armour and weapons for him.

Hector's wife Andromache knows that the return of Achilles threatens her husband, and she begs him not to fight. But Hector leaves to face his fate. Meanwhile, Achilles has become uncontrollable. The gods decide to re-enter the battlefield, so each of them can defend their side. Achilles has reached the River Scamander and it flows red with the blood of slain Trojan warriors. The river god Scamander then turns against Achilles, sweeping him away in the current. Hera panics and asks Hephaestus to save Achilles. Will he reach him in time?

Mount Olympus no longer resembles Mount Olympus. On the battlefield, Hector takes fright when he sees Achilles advancing towards him. When the duel between the two heroes finally takes place, Hector launches his spear at Achilles but the goddess Athena diverts its flight. Achilles then kills Hector, ties the dead body to his chariot and drags it around the city walls. When night falls, King Priam implores Achilles to hand over his son's body. Moved by the old king's request, Achilles agrees. That evening, Odysseus finally comes up with a plan that could give victory to the Greeks.

When the Trojans assume that the horse must be an offering to Athena, Priam has it brought into the city. At nightfall, the Greeks climb out of the horse, opening the gates of the city to the rest of their army, and start to massacre the Trojans. Menelaus is reunited with Helen. Paris fires an arrow into the heel of Achilles, who collapses to the ground. Zeus watches on helplessly and realises that men will no longer be able to believe in the gods after such a slaughter. With the city still burning, Odysseus sails away with his comrades, without any sense of triumph. Ten years of fighting. But who will remember the Trojan War?