In the tenth and final year of the Trojan War, Chryses, a priest of Apollo, attempts to ransom his daughter from Agamemnon, commander-in-chief of the Achaeans, who has taken her captive while on a raid. When Agamemnon treats him roughly and refuses the ransom, Apollo is angered and brings plague on the Achaeans. The Achaean prophet Calchas correctly identifies the cause of the problem, and he suggests giving the girl back with gifts to Apollo. Agamemnon demands that he be compensated for the loss of the girl, and Achilles, the greatest Achaean warrior, objects. The two men quarrel viciously. Agamemnon says he will take back Briseis, a captive woman who was given to Achilles as a prize for valor. Horribly dishonored, Achilles returns to his ships and refuses to fight. Agamemnon has Briseis taken from Achilles, and he returns Chryses' daughter to him. Apollo asks his mother, the goddess Thetis, to prevail on Zeus, king of the gods, to bring ruin on the Achaeans as long as Achilles does not fight for them. Zeus is indebted to Thetis, and he grants her request.

With Achilles out of the way, Hector, champion of the Trojans, drives the Achaeans back to their beached ships. The Achaeans build fortifications, but at the urging of the chieftains Agamemnon sends and embassy to ask Achilles to return to battle. Agamemnon offers rich prizes, but Achilles refuses the offer and remains withdrawn from battle.

The Achaean fortifications are breached, and many of the the greatest remaining Achaean warriors are wounded. Achilles beloved companion, Patroclus, begs Achilles to do something to help their fellow soldiers. He asks that he be allowed to put on Achilles' armor, so that the Trojans will think that Achilles has returned. Achilles grants the request, but warns Patroclus to return once he has driven the Trojans back from the ships. Patroclus drives the Trojans back all the way to their own city walls, but there Hector kills him with the help of Apollo. Hector strips his armor and puts it on himself, and the Achaeans barely manage to save Patroclus' body from desecration.

Achilles goes berserk with grief and rage. Thetis warns him that if he kills Hector, he will die soon afterward. Achilles accepts his own life as the price for revenge. He reconciles himself to Agamemnon, receives new armor, via his mother, forged by the smith of the gods, Hephaestus. He charges into battle, slaughtering Trojans left and right, routing the Trojan army almost single-handedly. He meets Hector, chases him around the city, and kills him easily. He then drags the body from the back of his chariot, running laps around the city of Troy so that the Trojans can watch as their champion's body is horribly desecrated.

Achilles returns to the Achaean camp, where he holds magnificent funeral games for Patroclus. He continues to abuse Hector's corpse. Zeus sends Thetis to tell Achilles that he must accept the ransom that Priam, king of Troy and father of Hector, will offer in exchange for Hector's body. Priam himself comes to see Achilles, the man who has slaughtered so many of his sons, and Achilles suddenly is reminded of his own father‹who, as Priam has, will outlive his most beloved son. He understands what he has done, and his rage and grief give way to compassion. He returns the body and offers a cease-fire so that the Trojans can bury Hector. With the word of Achilles as their guarantee, the Trojans take eleven days to give Hector a proper mourning and funeral. As the epic ends, the future is clear: Achilles will not live to see the fall of Troy, but the city is doomed nonetheless. All but a handful of her people will be slaughtered, and the city will be wiped off the face of the earth.