The Song of Achilles is told from the perspective of Achilles’ lover, Patroclus. Patroclus, who is a young Greek prince, grows up with a father disappointed by Patroclus’ mediocrity. When Patroclus is nine years old, his father takes him to Sparta where Patroclus presents himself as a suitor for Helen. Helen chooses her husband, Menelaus, from the suitors, and the rest of the men make an oath that they will defend her choice. 

After Patroclus accidentally kills a boy of noble birth, Patroclus’s father exiles him to Phthia where he befriends King Peleus’ son, Achilles. At first, Patroclus is resistant to friendship with Achilles, but eventually the two develop a bond. Patroclus becomes Achilles’ brother-in-arms who is sworn to him by blood oath and love. Patroclus follows Achilles to Mount Pelion where they are trained by the centaur Chiron. Achilles’ mother, the sea-nymph Thetis, expresses her dislike of Achilles and Patroclus’ relationship, and hates that Patroclus followed Achilles to Mount Pelion. One night when they are alone, Patroclus and Achilles make love.

Patroclus and Achilles are eventually called back to Phthia, where they learn that Paris of Troy has kidnapped Helen of Sparta. Patroclus remembers the oath he took as a child to defend Menelaus and Helen, and Patroclus worries that he will be drawn into the war. There is a prophecy that says the war against Troy is where Achilles will make his name famous and become godlike, but he will also die at war after the Trojan Hector dies. At first Achilles resists joining the fight, with Thetis even hiding him on the island of Scyros with King Lycomedes. At Scyros, Achilles masquerades as a girl to avoid the war. Lycomedes’ daughter, Deidameia, discovers Achilles’ disguise and secretly marries him, conceiving a child named Pyrrhus. When Odysseus and Diomedes arrive at Scyros and reveal Achilles’ true identity, they convince him to go to Troy. Patroclus follows, knowing that Achilles is his true love and he would rather die than be separated.

Achilles and Patroclus join the Greeks, led by Agamemnon, and go to war with the Trojans at Troy. Achilles wins multiple fights easily, demonstrating his superior warrior strength. When the Greeks raid the Trojans, Achilles claims a young girl named Briseis from the plunder, attempting to save her from Agamemnon’s violent lust. Patroclus and Briseis quickly become close companions. 

When Agamemnon offends the gods by refusing to return the daughter of a priest, the gods send a plague among the Greeks. Achilles tries to tell the Greeks that the reason for the plague is Agamemnon’s refusal to return the girl, but Agamemnon responds by taking Briseis away from Achilles and dishonoring him in front of the rest of the men. As a result, Achilles refuses to fight in Agamemnon’s ranks, believing himself to be superior. As the Greeks subsequently lose battles and soldiers die in scores, Patroclus adorns Achilles’ armor and takes his place in the war. Patroclus kills one of the strongest Trojan warriors, but is soon thereafter killed by Hector. Achilles, mad with grief, returns to war after Patroclus’ death and eventually kills Hector, parading Hector’s fallen corpse around in the process. King Priam of Troy visits Achilles one night and begs him to release Hector’s body so that he can have a proper funeral. Achilles agrees. 

As the war continues, Achilles kills more and more notable Trojans. He is finally killed by Paris, King Priam’s son who had taken Helen, when Paris shoots Achilles with an arrow. Afterward, Achilles’ son Pyrrhus enters the war. Pyrrhus rejects the idea of Achilles’ ashes being buried alongside Patroclus’, but Thetis arranges for their joint burial. In the conclusion of the novel, Patroclus describes looking at his and Achilles’ grave, and reuniting with Achilles in the underworld.