After an invocation to the Muses, the story launches in medias res towards the end of the Trojan War between the Trojans and the besieging Achaeans. Chryses, a Trojan priest of Apollo, offers the Achaeans wealth for the return of his daughter Chryseis, held captive by Agamemnon, the Achaean leader. Although most of the Achaean army is in favour of the offer, Agamemnon refuses. Chryses prays for Apollo's help, and Apollo causes a plague to afflict the Achaean army.

After nine days of plague, Achilles, the leader of the Myrmidon contingent, calls an assembly to deal with the problem. Under pressure, Agamemnon agrees to return Chryseis to her father, but decides to take Achilles' captive, Briseis, as compensation. Achilles furiously declares that he and his men will no longer fight for Agamemnon and will go home. Odysseus takes a ship and returns Chryseis to her father, whereupon Apollo ends the plague.

In the meantime, Agamemnon's messengers take Briseis away. Achilles becomes very upset, sits by the seashore, and prays to his mother, Thetis.[3] Achilles asks his mother to ask Zeus to bring the Achaeans to the breaking point by the Trojans, so Agamemnon will realize how much the Achaeans need Achilles. Thetis does so, and Zeus agrees.

Zeus sends a dream to Agamemnon, urging him to attack Troy. Agamemnon heeds the dream but first decides to test the Achaean army's morale, by telling them to go home. The plan backfires, and only the intervention of Odysseus, inspired by Athena, stops a rout.

Odysseus confronts and beats Thersites, a common soldier who voices discontent about fighting Agamemnon's war. After a meal, the Achaeans deploy in companies upon the Trojan plain. The poet takes the opportunity to describe the provenance of each Achaean contingent.