Orestes was born in Mycenae, the great citadel in the Argolid that was the richest and most powerful city in Greece, the son of Agamemnon and Clytemnestra. His father was the high king of the Greeks, the leader of the expedition against Troy, and his mother was a princess of Sparta, the daughter of Tyndareus and Leda, the woman who had laid two eggs and borne both divine and mortal children. When Agamemnon sailed for Troy, he left Clytemnestra in charge of Mycenae with a bard to guard her virtue, but the bard was murdered by Aegisthus, the cousin of Agamemnon who had been waiting ten years for his revenge. Aegisthus seduced Clytemnestra, and together they ruled Mycenae as king and queen while the war dragged on across the sea.

When Agamemnon returned from Troy, bringing with him the Trojan princess Cassandra as a concubine, Clytemnestra and Aegisthus were ready. They laid a purple carpet for the king to walk upon, a trap of honour that would lead him to the bathhouse where they had prepared a net and a sword. Agamemnon was murdered as he bathed, and Cassandra, the prophetess cursed by Apollo so that no one would ever believe her warnings, was killed beside him. Clytemnestra then turned her attention to the children. She sent Orestes, who was still a boy, into exile, and she imprisoned Electra, his elder sister, in the palace, marrying her off to a peasant to neutralise her as a threat. Orestes was taken to Phocis, to the court of King Strophius, who was an ally of Agamemnon, and there he was raised alongside Strophius's son Pylades. The two boys became inseparable, their friendship so deep that later Greeks used their names as a byword for perfect loyalty between men.

When Orestes came of age, Apollo, the god of light and prophecy, commanded him through his oracle at Delphi to avenge his father's murder. The command was absolute: Orestes must kill Clytemnestra and Aegisthus, or he would be cursed by the god himself. Orestes did not hesitate. With Pylades at his side, he returned to Mycenae in secret, disguised as a traveller, and made contact with Electra, who had never stopped mourning her father and dreaming of revenge. Together they plotted the deaths of the usurpers. Orestes found Aegisthus first and killed him with a sword-thrust, then turned to his mother. Clytemnestra, seeing her son with blood on his hands, bared her breast and begged him to remember that she had nursed him, that she had carried him in her womb, that no son should kill his mother. Orestes hesitated, but Pylades reminded him of Apollo's command, and Orestes struck the fatal blow. Clytemnestra fell, and the house of Atreus was avenged.

But the avenger became the condemned. The moment the sword fell, the Furies appeared, the Erinyes, the ancient avenging spirits who punish murder within the family. They were older than the Olympian gods, born from the blood of Uranus when Cronus castrated him, and their sole purpose was to hound the matricide to madness and death. They pursued Orestes across Greece, invisible to all but him, their breath hot on his neck, their torches burning his eyes, their voices screaming in his ears day and night. He wandered in madness, driven from city to city, until he reached Delphi and threw himself upon the altar of Apollo, begging for purification. Apollo sent him to Athens, to the court of the Areopagus, the hill of Ares, where the oldest tribunal in Greece sat in judgment upon the most terrible crimes.

The trial was the most famous in Greek myth. The Furies prosecuted Orestes, demanding his blood as the price for his mother's blood. Apollo defended him, arguing that a mother is only the nurse of the seed, while the father is the true parent, and that Orestes had obeyed a divine command. The court was divided, half for conviction and half for acquittal. Athena, the goddess of wisdom and the patron of Athens, presided over the court, and she cast the deciding vote. She voted for acquittal, and Orestes was freed. The Furies, enraged at being cheated of their prey, threatened to destroy Athens with plague and famine, but Athena persuaded them to accept a new role: they would become the Eumenides, the Kindly Ones, honoured spirits who protected the city and punished only those who were truly guilty. Orestes, purified and free, returned to Mycenae and reclaimed his father's throne, ruling as king of Argos and Mycenae until his death.