Pylades, Orestes, and the Old Man, Orestes's keeper, arrive at Mycenae at daybreak. They have come to exact revenge for the murder of Agamemnon, Orestes's father, as instructed by an oracle of Apollo. Electra, Orestes's sister, is heard sobbing within the house outside of which the three stand discussing how to execute their plan. Orestes wishes to greet her, but instead the Old Man leads him away to present an offering at his father's grave, as Apollo's oracle has instructed. Electra emerges from inside the palace gates, pouring forth her grief in a mournful address to the heavens and praying to the deities to help her exact revenge for her father's death. The chorus, which is comprised of the virgins of the palace, attempt to console Electra, but Electra, bemoaning the oppression she suffers at her mother's hands, her deep sorrow at her father's death, and her longing for Orestes' return, proves inconsolable.
Chrysothemis, Electra's younger sister, emerges from the palace with a funeral offering. She is unsurprised to find Electra mourning as usual outside the palace gates, and scolds her for it, urging her to get on with her life. She warns Electra that Aegisthus, their mother, Clytemnestra's, husband, intends to lock her away in a secluded room unless she pulls herself together, to which threat Electra rolls her eyes. Chrysothemis then explains her purpose in carrying funeral offerings. She has been sent to make an offering at Agamemnon's grave by Clytemnestra, who is terrified by a dream she had the night before that Agamemnon, the husband she murdered, returned and planted his scepter in the floor of the house. The scepter grew branches from which leaves grew and overshadowed the land. Electra successfully convinces her sister to throw away their mother's offering and instead to leave an offering of their own. Chrysothemis agrees and leaves to do so.
Clytemnestra approaches with a servant who also carries an offering. She chastises Electra for mourning in the streets, and the two have an argument about Agamemnon's murder. Clytemnestra maintains that it was a just murder, done as revenge for Agamemnon's sacrifice of their daughter, Iphigenia. Electra holds that the sacrifice was necessary, and that, regardless of this detail, Clytemnestra murdered Agamemnon out of lust for Aegisthus. After this angry altercation, Clytemnestra, standing beside an altar, prays to Apollo for wealth, longevity, and, in guarded terms, for the death of her son, Orestes, so that he might not return and disrupt her life. As soon as Clytemnestra has finished praying, the Old Man enters in the character of a Phocian, bearing a false account of Orestes's death. Clytemnestra is briefly touched by maternal feelings, yet delighted that her prayer has been answered; Electra is overcome with grief. Clytemnestra ushers the Old Man inside to receive her hospitality.
Chrysothemis returns from Agamemnon's grave, full of joy and sure that Orestes has returned, because at their father's grave she found a fresh offering of flowers and a lock of hair. Electra informs her sister that, to the contrary, Orestes is dead, and that is revenge is to be achieved, the sisters must take matters into their own hands. Chrysothemis rejects the idea as too risky, and leaves Electra to her grieving. Orestes then approaches with Pylades in the character of a Phocian, carrying an urn that, it is pretended, contains Orestes' ashes. Electra's despair is heightened, and she takes the urn and laments over it at length. Orestes, visibly affected by the sight of his sister's suffering, reveals himself to her, displaying their father's ring as proof of his identity. Electra's grief is immediately replaced with joy, and she is in the middle of celebrating when the Old Man emerges from within the palace and hurries Orestes inside to kill Clytemnestra.
Electra remains outside, watching for Aegisthus so that he might not return unobserved. She listens excitedly to the sounds of Orestes killing Clytemnestra. Orestes, having successfully killed his mother, returns to Electra to tell her the news, but he hurries back inside as they see Aegisthus approaching. Aegisthus asks Electra where the Phocians with news of Orestes's death have gone, and she points inside the house. He demands that the doors and gates of the palace be opened, so that the citizens can see what he thinks is Orestes's corpse. Orestes brings the corpse of Clytemnestra, covered with a sheet, to Aegisthus, and when the body is uncovered, Aegisthus realizes that he has been trapped and that death is upon him. Orestes orders Aegisthus inside so that he might kill him in the exact spot where, years before, Aegisthus killed Agamemnon.