The play Agamemnon (Ἀγαμέμνων, Agamemnōn) details the homecoming of Agamemnon, King of Argos, from the Trojan War. Waiting at home for him is his wife, Clytemnestra, who has been planning his murder, partly as revenge for the sacrifice of their daughter, Iphigenia, and partly because in the ten years of Agamemnon's absence Clytemnestra has entered into an adulterous relationship with Aegisthus, Agamemnon's cousin and the sole survivor of a dispossessed branch of the family (Agamemnon's father, Atreus, killed and fed Aegisthus's brothers to Aegisthus's father, Thyestes, when he took power from him), who is determined to regain the throne he believes should rightfully belong to him.
The play opens to a watchman on top of the house, reporting that he has been lying restless there "like a dog" (kunos diken) for a year, "for so rules the expectant manly-willed heart of a woman" (that woman being Clytemnestra awaiting the return of her husband, who has arranged that mountaintop beacons give the signal when Troy has fallen). He laments the fortunes of the house, but promises to keep silent: "A huge ox has stepped onto my tongue." However, when Agamemnon returns, he brings with him Cassandra, the enslaved daughter of the Trojan king, Priam, and a priestess of Apollo, as his concubine, further angering Clytemnestra.
From the silence of the watchman the chorus begin with the great parodos, which as Kitto expressed it ['It lays down the intellectual foundation of the whole trilogy'], bears the weight of the trilogy . . . Through descriptions of the past, hopes and fears for the future, and statements of the present (which together constitute the narrative) this song develops a series of tensions . . .[it] opens with the narrative of events leading towards the Trojan expedition
The central action of the play is the agon between Clytemnestra and Agamemnon. She plays the loving, waiting wife and attempts to persuade Agamemnon to step on a purple (sometimes red) tapestry or carpet to walk into "his" palace as a true returning conqueror. The problem is that this would indicate hubris on Agamemnon's part, and he is reluctant. Eventually, for reasons that are still heavily debated, Clytemnestra does persuade Agamemnon to cross the purple tapestry to enter the oikos, where, according to her later account, she kills him in the bath: she ensnares him in a robe and as he struggles to free himself she hacks him with three strokes of a pelekus.
While Clytemnestra and Agamemnon are offstage, Cassandra, who had heretofore been silent, is suddenly possessed by the god Apollo and enters a tumultuous trance. Gradually her incoherent delirium starts making some sense, nouns and adjectives lining up to form the rudiments of meaning, and she engages in anguished discussion with the chorus, whether she should enter the palace, knowing that she too will be murdered. Cassandra has been cursed by Apollo for rejecting his advances at a feast celebrating the opening of his temple in Corinth. He has given her clairvoyance so that she can foresee future events, but he has cursed her so that no one who hears her prophesies will believe them until it's too late. In Cassandra's soliloquy, she runs through many gruesome images of the history of the House of Atreus as if she had been a witness of them (though she is too young to have seen them), including betrayal, murder, cannibalism, and rude language. She eventually enters the palace, knowing that her fate is preordained and unavoidable. The chorus, in this play a group of the elders of Argos, are left bewildered and fearful, until they hear the death screams of Agamemnon and frantically debate on a course of action.
A platform is then rolled out by the palace servants displaying the butchered and dismembered corpses of Agamemnon and Cassandra, along with Clytemnestra brandishing the bloodied axe of the Cyclops, and defiantly explaining her action. Agamemnon was murdered in much the same way an animal is killed for sacrifice: with three blows, the last strike accompanied by a prayer to a god. Cassandra was killed with only two blows, omitting the prayer. She is soon joined by Aegisthus, Agamemnon's dispossessed cousin and her lover, now the king, strutting out and delivering an arrogant speech to the chorus, who nearly enter into a brawl with him and his guard. However, Clytemnestra halts the dispute by swinging the axe wildly, saying that "There is pain enough already. Let us not be bloody now." The play closes with the chorus reminding the usurpers that Orestes, the son of Agamemnon, will surely return to exact vengeance.