The Libation Bearers begins at the burial mound of Agamemnon (the king of Argos and the leader of the Greeks during the Trojan War), who died in the play’s prequel—Agamemnon—at the hands of his wife Clytemnestra and her lover Aegisthus. Orestes, Agamemnon’s and Clytemnestra’s son, has returned in secret from exile with his companion Pylades. He cuts a lock of his hair to honor his father and reveals his intention to avenge Agamemnon’s death.
Hearing others approaching, the men hide behind the tomb. As they do, Orestes’ sister, Electra, enters. A Chorus of slave women, bringing offerings to Agamemnon’s tomb from Clytemnestra, attends her. Electra mourns her father and curses her mother. As she prays for her brother, and for vengeance, Electra notices Orestes’ hair and compares it to her own. She then follows his footprints and the two joyfully reunite. Orestes reveals to the Chorus and to Electra that the god Apollo has sent him to Argos to avenge Agamemnon’s death. The siblings and the slave women pray to the gods for vengeance, and reiterate their loyalty to the deceased Agamemnon.
Before leaving the grave, Orestes asks the Chorus why Clytemnestra sent offerings to her hated husband’s tomb. The Chorus explains that the queen had a terrible dream in which she gave birth to a serpent that then killed her. Orestes wonders if he himself is the serpent. He then relates his plan: he and Pylades will disguise themselves as guests, and when inside the palace, they will murder both Aegisthus and Clytemnestra.
The Chorus meditates on the bravery of men, and the treachery of women. They mention examples of wicked women before turning in particular to Clytemnestra, asserting that the gods detest creatures such as her and that Justice and Fate will soon punish her.
Now disguised, Orestes and Pylades wait at the gates of the palace. When they finally enter, Clytemnestra meets them. Orestes lies, telling her that her son is dead. Clytemnestra briefly (and falsely) laments before resolving to find Aegisthus. The Chorus reemerges, praying to the gods to guide Orestes. As they do, Orestes’ old nurse, Cilissa, enters, lamenting the prince’s supposed death and looking for Aegisthus. The Chorus urges her to tell him of Orestes’ demise in order to put him at ease. Although confused, Cilissa agrees to do as they ask.
As the Chorus prays again, Aegisthus enters, wondering if Orestes is really dead, and resolving to question the mysterious travelers himself. He exits, and then the Chorus hears a scream coming from inside the palace—Orestes has murdered Aegisthus. A wounded servant rushes in to warn Clytemnestra, who emerges and calls for an axe to defend herself. Orestes, however, is too quick for her, and despite the fact that his mother both pleads and threatens (warning that she will put a “curse” on him if he kills her), he drags her within the palace and murders her on top of Aegisthus’ body.
A manic Orestes winds the corpses in the shroud in which they had trapped Agamemnon to murder him. Orestes incoherently laments his father’s death and his own guilt, stating that he must now exile himself from Argos. As the Chorus tries to reassure him, Orestes sees the Furies—vengeful goddesses who mean to punish him for Clytemnestra’s death. Terrified, he runs offstage, as the Chorus wonders whether the endless cycle of vengeance will ever come to a close.