In The Libation Bearers, the old blood-for-blood way continues to prevail. Orestes, the son of Agamemnon and Clytemnestra, has arrived at the palace, having been away for some time. He and his cousin Pylades are standing at Agamemnon’s grave. They hear someone approaching and hide. Soon Orestes’ sister Elektra arrives with a chorus of mourning slaves (the libation bearers) sent by the queen, presumably in hopes of warding off harm to herself as a result of her murderous act.

Orestes and Pylades now reveal themselves and ask why the libation bearers are there performing their ritual. The chorus reveals that Clytemnestra has had a nightmare in which she gave birth to a snake which then sucked a mixture of milk and blood from her breast. Orestes says he is that snake, and brother and sister plan revenge.

Orestes and Pylades pretend to be travelers and request a place to stay at the palace. They tell Clytemnestra that Orestes is dead and, much relieved, she calls for Aighistos. When the usurping king arrives, Orestes kills him and is faced with the central problem of the play. Revenge for the killing of a father and king—fratricide and regicide—is required by the unwritten rules of the old blood-for-blood way. But in this case there is an impossible problem, as the object of revenge must be his mother, meaning Orestes must commit the forbidden sin of matricide. As is so often the case in tragedy, the hero is caught between conflicting demands, and there is no way out. Orestes kills Clytemnestra as he and Pylades, and certainly Elektra—the driving force behind the planned action—believe he must, and as the old law demands. Almost immediately he is set upon by the guardians of the old way, the Furies, known to the Greeks as the Eumenides or Erinyes, who charge the young hero with matricide. Orestes flees in horror, and the chorus says the tragedy of the House of Atreus will continue.