The collective name given to the three Greek tragedies (trilogy) by Aeschylus on the story of Agamemnon, Clytemnestra, and Orestes, produced at Athens in 458 BC when it won the dramatic competition. It is the only trilogy that survives complete. The plays are "Agamemnon," "The Libation Bearers," and "Eumenides" (meaning kindly ones, a euphemism for the Furies).
The story is taken from the mythical history of the descendants of Atreus in which crime led to further crime through several generations. "Agamemnon" opens in an atmosphere of hope mingled with foreboding, as the watchman on the roof of Agamemnon's palace in Argos looks out for the signal beacon to announce the fall of Troy. After the signal is seen, the news is confirmed by the arrival of a herald. Agamemnon's wife Clytemnestra appears jubilant, but the chorus of Argive elders recalls Agamemnon's sacrifice of his daughter Iphigeneia to enable the Greek fleet to set sail, and brood over the possible consequences. Agamemnon arrives bringing with him the captured Trojan princess Cassandra, his concubine. Clytemnestra treacherously welcomes him and then leads him into the palace. Cassandra, who has not spoken up to this point, is now moved to frenzied prophecy, foresees Agamemnon's murder and her own, as well as having a vision of the past crimes of the house, and utters a lament. She too enters the house, knowingly going to her death. Thc cries of the dying Agamemnon are heard. The interior of the palace is revealed, with Clytemnestra exulting over the bodies of the two victims. She answers the elders' reproaches by citing as justification Agamemnon's sacrifice of Iphigeneia. Aegisthus, her lover, appears, and subdues the elders with threats of force. The latter can only hope that one day Agamemnon's son Orestes will avenge King Agamemnon.
In "The Libation Beareres," Orestes, the son of Agamemnon, after years of exile, returns to Argos with his friend Pylades, to avenge his father on the god Apollo's instructions; he comes to his father's tomb and dedicates on it a lock of his hair. The two draw aside while Electra, Orestes' sister, and a chorus of Argive women approach to pour libations on the tomb by order of Clytemnestra, who has been disturbed by ominous dreams. Electra recognizes the lock of hair and footprints nearby as strikingly similar to her own; her brother reveals himself and a reunion takes place (a scene thought by some to be mocked by Euripides in his Electra, where Electra rejects these proofs of identity). Electra and Orestes join in an impressive invocation of their father's dead spirit calling upon his aid in their pursuit of ven-geance. Orestes and Pylades, disguised as travellers bringing news of the former's death, enter the palace. Aegisthus is summoned and on his arrival is killed by Orestes. Clytemnestra pleads with her son for her life, and for a moment Orestes falters; but Pylades, in his only speech, reminds him of Apollo's command, and Orestes drags her into the palace and kills her. While he is justifying his action he sees avenging Furies arrive to haunt him, and he flees from them.
"Eumenides" opens to show Orestes as suppliant at the shrine of Apollo in Delphi. The Furies, forming the chorus, are asleep around him. Orestes is promised protection by Apollo, who tells him to go to Athens to seek justice from the goddess Athena. After he leaves, the ghost of Clytemnestra stirs up the Furies to pursue him. The scene changes to the front of Athena's termple on the Acropolis at Athens. Athena, having heard the pleas and justifications of the Furies and of Orestes, refers the judgement to a tribunal of Athenian citizens acting as judges (i.e. the historical Areopagus court for judging cases of homicide, of which this episode was the legendary foundation). The Furies prosecute, and Orestes defends himself. Athena votes with the other judges, and the votes are found to be equally divided. Athena therefore declares that in the future, when the votes are equal the defendant is to be acquitted (as was Athenian law). The Furies are indignant, but conciliated by Athena's promise of a permenanat home in her city and honour in the new role as beneficent powers.
The Oxford Companion to Classical Literature, edited by M. C. Howatson. New York: Oxford University Press, 1989, pages 398-399.