In Agamemnon, he returns from defeating Troy to his wife Clytemnestra. His wife is having an affair with Aegisthus. Agamemnon has brought back a slave names Cassandra who can divine the future with the curse that no one will believe her. She foresees Agamemnon's death. Cassandra murders him in the tub with an axe.
Clytemnestra and Aegithus murder Cassandra and get into an argument with the Chorus over the whole thing. The Chorus forewarns them that Agamemnon's son, Orestes will come for them.
In The Libation Bearers, Clytemnestra and Aegithus now rule Argos. They have banished Orestes out of fear of what he will do to them. THe play begins with Clytemnestra waking from a dream about a snake biting her breast. Fearing it is an omen, she orders her daughter, Electra, to pour waters of blessing over Agamemnon's grave in the hopes to appease the gods.
Orestes returns and meets Electra at Agamemnon's grave. Hearing about the prophetic dream, the two decide to kill their mother and take back the throne The Chorus is on their side because they are comprised of laves and former Argonauts who favor Orestes and Electra.
Orestes recruits Pylades as a sidekick. They pretend to be travelers and tell the guards that Orestes has died. A maid is sent to get Aegithus, but the Chorus convinces her to make him come alone. Orestes kills him. When Clytemnestra sees this, he siezes her by the throat and kills her. Before she dies she warns him of the curse it will bring upon him. Sure enough, when she dies the Erinyes, or Furies, appear and explain that the sin of killing your mother is greater than that of killing your husband. Orestes flees Argos.
In The Eumenides, Orestes takes sanctuary in the Temple of Apollo in Delphi. Orestes is found in the temple surrounded by sleeping Furies. Apollo, unable to stop them, put them to sleep. Orestes continues on to Athens under the protection of Hermes.
At this point Clytemnestra's ghost wakes the Furies and urges them on. The Erinyes hunt him by tracking the blood of his mother, which Orestes leaves with every footstep. THey surround him and pleads for Athena , goddess of justice, to intervene.
Oreses stands before 12 Athenians and Athena presides over the trial. It's basically Apollo and Orestes vs the Erinyes for Clytemnestra. It's a split decision, and Athena sides with Orestes. To calm the fury of the Erinyes, she renames them the Eumenides or kindly ones. It is said from that point on that hung juries will always decide in favor of the defendant.