The Murder of Agamemnon
Electra was absent from Mycenae when her father, King Agamemnon, returned from the Trojan War to be murdered, either by Clytemnestra's lover Aegisthus, by Clytemnestra herself, or by both. Clytemnestra had held a grudge against her husband Agamemnon for murdering their eldest daughter, Iphigenia, as sacrifice to Artemis, so he could send his ships to fight Troy for the Trojan war. When he came back he brought with him his war prize, Cassandra, who had already borne his twin sons. Aegisthus and Clytemnestra killed Agamemnon upon his arrival, and they killed Cassandra as well. Eight years later, Electra was brought from Athens with her brother, Orestes. (Odyssey, iii. 306; X. 542).

According to Pindar (Pythia, xi. 25), Orestes was saved by his old nurse or by Electra, and was taken to Phanote on Mount Parnassus, where King Strophius took charge of him. When Orestes was 20, the Oracle of Delphi ordered him to return home and avenge his father's death.

Electra and Orestes, from an 1897 Stories from the Greek Tragedians, by Alfred Church
According to Aeschylus, Orestes saw Electra's face before the tomb of Agamemnon, where both had gone to perform rites to the dead; a recognition took place, and they arranged how Orestes should accomplish his revenge. Pylades and Orestes killed Clytemnestra and Aegisthus (in some accounts with Electra helping).

Before her death, Clytemnestra curses Orestes and the Erinyes or Furies, whose duty it is to punish any violation of the ties of family piety, come to torment him. They pursue him, urging him to end his life. Electra was not hounded by the Erinyes. Orestes took refuge in the temple at Delphi. It is said that a priestess found him, covered in blood, and with the Furies flying around him. The priestesses washed him with pig blood to purify him. Once purified he traveled to Athens to seek Athena.

Athena (also known as Areia) received him on the Acropolis of Athens and arranged a formal trial of the case before twelve Attic judges. The Erinyes demanded their victim; he pleaded the orders of Apollo; the votes of the judges were equally divided, and Athena gave her casting vote for acquittal.

In Iphigeneia in Tauris, Euripides tells the tale somewhat differently. He claims that Orestes was led by the Furies to Tauris on the Black Sea, where his sister Iphigeneia was being held. The two met when Orestes and Pylades were brought to Iphigeneia to be prepared for sacrifice to Artemis. Iphigeneia, Orestes, and Pylades escaped from Tauris. The Furies, appeased by the reunion of the family, abated their persecution.