Chryses, who is a priest of Apollo, travels to the Achaian camp to plead for the release of his daughter Chryseis, who was taken captive during a Greek siege and awarded to Agamemnon as a spoil of war. Although Chryses has brought numerous offerings as payment for his daughter's release, Agamemnon declines to accept them and dismisses Chryses.
In retaliation for the mistreatment of his priest, Apollo inflicts the Greeks with a deadly disease that causes the loss of numerous lives. A meeting is convened to figure out how to put an end to the epidemic. Following the guidance of a prophet, the Greeks conclude that the only solution to halt the plague's devastation is to give back Chryseis.
Agamemnon, on the other hand, refuses to surrender his prize and insists that he must receive another person's prize as compensation. He demands Briseis, the woman awarded to Achilles during the same siege. Achilles is so furious with Agamemnon's action of taking Briseis that he immediately withdraws himself and his army from the war with Troy. He also beseeches his mother, the goddess Thetis, to intercede with Zeus on his behalf to avenge the injustice. Zeus agrees to support the Trojans in their assault on the Achaians, demonstrating to Agamemnon that Achilles is an important figure who is necessary for victory in the battle.
Agamemnon assembles the remainder of his troops for a colossal assault on the Trojans. The initial day of combat commences with a one-on-one battle between Paris and Menelaus, and a ceasefire is established among the remaining armies. Following the fight, in which Paris is removed from the battle by Aphrodite, the truce is violated by Pandarus, a Trojan, and the two forces engage in bitter fighting. At the conclusion of the day, another duel occurs, this time between Ajax and Hector, but it is terminated before a winner can be determined. The two groups withdraw, and the Achaians construct a barrier around their encampment to safeguard their location and their vessels.
When the fighting resumes, Zeus helps the Trojans achieve a significant triumph over the Achaians, making their victory appear certain. Upon this occurrence, Agamemnon gathers his leaders and confesses that he was wrong to take Briseis from Achilles. He consents to return Briseis, along with a vast amount of riches and a sworn declaration that he did not sleep with her, in exchange for Achilles' return and assistance in fighting with the Achaians. Odysseus, Ajax, and Phoenix relay this message to Achilles, who receives them warmly but declines to reconcile with Agamemnon.
On the following day, the Achaians resume their battle and display great skill. However, as the day progresses, many of their top warriors sustain injuries and are forced to withdraw from the fight. Among those injured are Agamemnon, Diomedes, Odysseus, Eurypylus (or Eurypylos), and Machaon. Consequently, the only remaining hero fighting for the Achaians is Ajax.
Afterward, Hector leads a powerful offensive by the Trojans, successfully breaching the Achaian wall and pushing their way toward the ships. As the Trojans attempt to set fire to the Achaian vessels, divine intervention occurs and the gods rescue the Achaians, saving them from near-certain destruction. At this point, both Achilles and his comrade Patroclus (or Patroklos) become anxious about the fate of the Achaian army. Despite still refusing to join the fight himself, Achilles sends Patroclus, outfitted in his own armor, along with a group of soldiers to defend the ships.
Due to Patroclus and his troops being well-rested and full of energy, they effortlessly push the exhausted Trojans back towards the city wall. Patroclus displays great bravery and performs numerous acts of valor, but he becomes overconfident and is ultimately slain by Hector. Hector seizes the renowned armor of Achilles from Patroclus, and a brutal struggle ensues over his body. Although the Achaians succeed in recovering Patroclus' body, the tide of the battle has shifted in favor of the Trojans, and the Achaians are forced to retreat.
After learning of his comrade's demise, Achilles becomes furious with Hector but is unable to engage in battle without his armor. Nevertheless, the gods transform him, and when he appears on the battlefield, the Trojans retreat, and the Achaians are able to withdraw. With the aid of the god Hephaestus (or Hephaistos), Achilles' mother Thetis procures immortal armor for him, and Achilles declares an end to his dispute with Agamemnon in front of the gathered Achaians.
On the following day, the Achaians succeed in forcing the Trojans to retreat within their city walls, largely due to Achilles' heroic feats. However, Hector refuses to retreat and instead pledges to confront Achilles directly. Despite his initial bravado, Hector loses his nerve and attempts to flee. Achilles chases him twice around the city walls before Athena deceives Hector, stopping his flight. The two engage in a duel, resulting in Hector's death, and Achilles drags his body by the feet behind his chariot all the way back to the Achaian encampment.
Achilles proceeds to organize a series of athletic contests in honor of Patroclus, with generous prizes being awarded to the winners. Patroclus' body is deeply lamented and then cremated on a grand pyre. In his anguish over his friend's death, Achilles has been shamefully mistreating Hector's remains, but the gods have prevented any further desecration. Guided in secret by the gods, Priam seeks out Achilles to offer a substantial ransom in exchange for his son's body. Moved by Priam's plea, Achilles takes pity on him and returns the body to the Trojans. Hector is then given a proper burial by his people.