Many years after king Agamemnon's murder at the hands of his wife Clytamnestra and her lover Aigisthos, his son Orestes returns home with Pylades to mourn at his grave. Orestes has been living in exile and has come back to Argos in secret, sent by an oracle of Apollo. His mission is to exact vengeance for Agamemnon's death upon his murderers. Apollo has threatened him with horrible punishments, including leprosy and further exile, if he does not agree to accept this quest.
While standing at Agamemnon's grave, Orestes meets up with his sister Electra, whom he has not seen since he was a child. There is a protracted recognition scene and then a subsequent joyful reunion. Electra explains that she was sent to the grave by their mother, Clytamnestra, to bring libations to Agamemnon in the hope she could quiet the source of Clytamnestra's terrible dreams. Orestes and Electra, encouraged by the chorus, discuss how much they love their father and hate their mother, and pray together in order to invoke the spirit of Agamemnon to come to their aid in their quest for vengeance.
Orestes and Electra engage in wishful thinking about how their father could have lived, but the chorus urges them to focus on the present and to act on their anger. Together, Orestes and Electra plot to avenge Agamemnon's death. With the eager support of the chorus, Orestes concocts a plan wherein he will gain admittance to the palace and kill Aigisthos on the throne. Electra and the chorus are complicit, and disappear back into the palace.
Unexpectedly, Clytamnestra comes to the door when Orestes knocks, thus forcing him to fabricate a story about his origins. He claims to be a stranger bearing sad news of the death of Orestes. Clytamnestra laments, and sends Cilissa, Orestes's old nurse, to tell Aigisthos to come with his bodyguard to hear the news.
The chorus intervenes, intercepting Clytamnestra's message, and tells Cilissa to compel Aigisthos to come alone, without his guard. Although she does not understand why the chorus seems so gleeful, since she assumes that Orestes is dead, Cilissa does as she is told. Aigisthos appears briefly on stage, after which he goes back into the palace to meet Orestes. His death is announced by his servant, who cries out for Clytamnestra to come and see what's happening.
Alarmed at all the shouting, Clytamnestra appears and immediately realizes that something is horribly wrong. The doors open, and she sees Orestes over the fallen body of Aigisthos. The climax of the play follows, as Orestes resolves to carry out his vengeance on his mother. He hesitates at the crucial moment, however, when Clytamnestra bares her breast to him and implores him to respect their filial bonds.
Pylades steps in at this moment, and reminds Orestes of his duty to Apollo. Orestes regains his resolve and deflects all of Clytamnestra's pitiful arguments. He stabs her, and the chorus rejoices. He wraps the two bodies in the same shroud in which Agamemnon was killed, and announces to the world that he has carried out the commands of Justice.
However, now that the deed is finally done, Orestes falls victim to the Furies's retributive violence. He goes mad and flees the stage in the direction of Delphi, where he will seek refuge at Apollo's shrine. The chorus despairs at the end of the play that the cycle of bloodshed has not stopped with Orestes's action, but continues ever still.