When King Agamemnon returns from the Trojan War with his new concubine, Cassandra, his wife Clytemnestra (who has taken Agamemnon's cousin Aegisthus as a lover) kills them. Clytemnestra believes the murder was justified, since Agamemnon had sacrificed their daughter Iphigenia before the war, as commanded by the gods. Electra, daughter of Agamemnon and Clytemnestra, rescued her young, twin brother Orestes from her mother by sending him to Strophius of Phocis. The play begins years later when Orestes has returned as a grown man with a plot for revenge, as well as to claim the throne.
Orestes arrives with his friend Pylades, son of Strophius, and a pedagogue, i.e. tutor. Their plan is to have the tutor announce that Orestes has died in a chariot accident, and that two men (really Orestes and Pylades) are arriving shortly to deliver an urn with his remains. Meanwhile Electra continues to mourn the death of her father Agamemnon, holding her mother Clytemnestra responsible for his murder. When Electra is told of the death of Orestes her grief is doubled, but is to be short-lived.
After a choral ode Orestes arrives, carrying the urn supposedly containing his ashes. He does not recognize Electra, nor she him. He gives her the urn and she delivers a moving lament over it, unaware that her brother is in fact standing alive next to her. Now realizing the truth, Orestes reveals his identity to his emotional sister. She is overjoyed that he is alive, but in their excitement they nearly reveal his identity, and the tutor comes out from the palace to urge them on. Orestes and Pylades enter the house and slay Clytemnestra. As Aegisthus returns home, they quickly put her corpse under a sheet and present it to him as the body of Orestes. He lifts the veil to discover who it really is, and Orestes then reveals himself. They escort Aegisthus off set to be killed at the hearth, the same location Agamemnon was slain. The play ends here, before the death of Aegisthus is announced.