The basic story is as follows. In the Iliad, Patroclus is Achilles's best friend. Enraged by the Trojan assault on the Greeks, which has almost reached their camp, Patroclus convinces Achilles to let him wear his armor and lead Achilles's army into battle. Achilles agrees, but he makes Patroclus promise to leave the fight after the Trojans have been beaten back. Patroclus, however, chases the Trojans back to the walls of Troy, slaying many, including Sarpedon, son of Zeus. Hector, the leader of the Trojans, mistakes Patroclus for Achilles and kills him.
The actions of all the heroes involved in this story are determined by the gods. Patroclus's death is foretold by Zeus, who says that Hector will be killed after he slays Patroclus. Apollo plays a significant role in the fighting, filling the Trojans with both strength and fear at appropriate moments and causing Patroclus to "lose his wits" at the moment when he decides to chase after the retreating Trojans rather than honor his promise to Achilles and retire from battle. Zeus decides that Patroclus must die to avenge the death of Sarpedon; Apollo secretly wounds Patroclus, and Hector kills him with his spear, in fulfillment of prophecy.
The death of Patroclus is the turning point in the war. Achilles, devastated by the death of his friend, returns to battle, kills Hector, and defeats the Trojans.