In the immediate aftermath of the fall of Troy, the play opens at King Agamemnon's palace in Argos with the lonely Watchman's soliloquy. From the roof of the palace, the Watchman begs the gods for respite from his interminable watch. The stars, his sole, plentiful and steadfast, companions seem to him like so many "dynasties" revolving in endless cycles, waxing and waning, moving out of winter into summer and back again. What he wishes, in short, is rest.
He relates how he has been obliged by the queen to keep watch for a fire. Further he cannot sleep for restless fear. In his musings he hints of a great bygone woe, "the pity of this house," which he hopes will soon be redeemed. The flames, he says, would presage positively. Far off in the distance, then, a light glows, and the Watchman spies a messenger's blaze that hails the fall of Troy. He draws a joyous analogy to a sunrise. The soliloquy closes with the Watchman hopeful that his king will return home, since the house, he says, has too long wallowed in a dismal sadness.
The senescent Chorus enters and begins its recapitulation of the commencement of the Trojan war tens years previous: the call to action, the deploying of the one thousand ships, the loss of so many young Argive lives. They go on to explain that the devastating fall is the exacting of a procrastinated punishment by angry gods upon the transgressors, mainly, Paris and Helen.
Clytaemestra enters and the Chorus continues. They inquire about the bonfires, sacrifices, and oblations the queen has ordered to be executed throughout the city, to all the gods. She is mute.
The Chorus recalls the omen, interpreted by a seer, of the hare tore open while still "ripe, bursting with young unborn yet." We are made aware that "the secret anger" which "remembers the child that shall be avenged," refers to anger over the sacrifice by Agamemnon of his maiden daughter Iphigeneia. Next, the disapproving Chorus outlines Iphigeneia's sacrifice at the hands of her battle-impassioned father, though without reaching its climax. What is to come, the future, the action of the play, apparently lies in the hands of Clytaemestra.
Clytaemestra announces victory at Troy. The leader of the Chorus, understandably skeptical, questions her source. She cites the concatenation of fires, beginning with the one in Troy, relayed across watchman at various posts, ending with our Watchman. Asked to unfold more of the story, the queen imagines the plundering of Troy at the moment, and warns the Chorus that the men should perpetrate no sacrilege, should maintain reverence for the foreign city's gods and citizens, lest in the act of despoiling, they despoil themselves before the journey home.
The Chorus interprets the news as the divine justice of Zeus on Paris, for his having stolen Helen away from Menelaus, her husband. There is no escape, they say, from perdition. But justice has come at great cost, and the lives of young men burn to dust in the flames - the people of Argos hate the war. The several Chorus voices its skepticism over the signal once again, displaying a jadedness. Then the Herald, a warrior, appears with word of Agamemnon's imminent arrival. He voices how terrible was his homesickness and how sweet its new relief. Following this news, Clytaemestra reminds the Chorus of its haughty attitude toward her "womanish" credulity, then openly proclaims her long, chaste fidelity to her husband. She moves backstage to make ready for his return.
An inquiry is made by the Chorus to the Herald as to Menelaus' state and whereabouts. It turns out he has disappeared in a terrible storm at sea. The Herald exits after narrating the storm. The Chorus, left alone on stage, muses again on the lamentable results of Helen and Paris's marriage. Daring is recorded as the undesirable offspring of aged Pride. Soon, Agamemnon, with Cassandra, a captive soothsayer, beside him, enter in a chariot. The leader of the Chorus admits to the king that, although he had despised of his decision to pursue Helen at all costs, he wholeheartedly welcomes his return. Agamemnon is eager to give thanks to the gods for his triumph.
Speaking to her husband in front of an assembly of Argive citizens, Clytaemestra relates how trying her wait has been in lieu of myriad tales of wounds and death to Agamemnon, and she implores the disgruntled audience to patience, to maintain the council and order. She greets Agamemnon in full grandiloquence, and the king is asked to step into his home on tapestries of crimson unfurled at his wife's command. But he refuses. I am a only a man, he says, a mortal, and will not support being honored like a god. The spouses clash over this, and Agamemnon is shown as a hard, unyielding man. Clytaemestra tries several different approaches to get him to accept her invitation. Her behavior is suspicious. After some more provocative words, however, Clytaemestra finally persuades Agamemnon to tread against his better judgment. He does so barefoot, as a human, but there is still something ominous in this. They enter the house.
The Chorus meditates on its uncured anxiety. Sight of Agamemnon has brought only more of the doleful dread and morbid fear. The Chorus cannot forget the injustice of the past, and neither, they are sure, can the gods. Sick at heart, they await the inevitable flow of blood. Clytaemestra reappears and orders the strangely mute Cassandra out of the chariot to worship at their altar. When the girl stays put, Clytaemestra leaves, not wishing to waste anymore of her time. Cassandra cries out insanely to Apollo, who the Chorus notes is not a god of lamentation, and utters abstruse prophecies about infanticide, fatal baths, and a murderess in the house. The Chorus believes she merely augurs her own death. They discuss the origin of her gift - and her curse, which auditors forever be incredulous of her veracious forecasts. As predicted, her most clear and disturbing divination, "you shall look on Agamemnon dead," is misunderstood. Finally, she sees in the future a son (Orestes) who will eventually come to murder the mother (we must assume this is Clytaemestra, although her name appears in none of the prophecies) that kills his father. Cassandra then enters the house, having resolutely accepted it also as her tomb.
From inside the house a sudden cry is heard. Agamemnon has been stabbed in the bathtub. The Chorus, in a panic, disintegrates, and the individual members speak frantically among themselves. They show themselves to be cowards, and Agamemnon cries out again before they even decide to take action. At once the doors of the palace swing open and behold! there lie Agamemnon and Cassandra, dead, with Clytaemestra standing over them. She describes to them, cold-bloodedly, it seems, the gruesome facts of her seduction, entrapment, and murder of the king who, she says, brought them all so much pain. She struck him thrice and gloried in the warm sputters of blood that shot from the wounds. She is remorseless; the Chorus is appalled at her brutality. The old men renounce her immediately. Next, Clytaemestra tries to justify her action as righteous, as ordained by the gods, retribution for the slaughter of her daughter. She portrays herself as an instrument of divine causality, of destiny. The Chorus will not hear of it and continues to wonder how they should mourn the dead king. The meaning of his death is still uncertain. Normally there would be a public lament for the fallen hero. Clytaemestra indicts all Argos in her action and declares that her husband shall not be mourned. Essentially they are debating culpability; that is, whether Clytaemestra's actions were divinely caused, or whether what she did was motivated by a base, human desire for revenge. In the end, having no other recourse, the fretting Chorus must agree with Clytaemestra.
But, just then, Aegisthus, exiled son of Thyests and the queen's secret lover bursts into the palace crying that he hatched the plot - he helped murder Agamemnon in revenge for his father (His father, Thyestes, was tricked by Agamemnon's father into devouring his two sons, Aegisthus' brothers). The Chorus predicts his downfall as before they had presaged Clytaemestra's. They accuse him of womanly cowardice for not having killed Agamemnon himself. Tyrannical Aegisthus then threatens the old men and the state with torture and bondage. When the Chorus, insolent to Aegisthus's boasting, rises up, Clytaemestra intervenes. Orestes is spoken of as the only hope for Argos. Deaf to the impotent gibes of the Chorus, Clytaemestra reminds her lover and new king that they now have the power. They enter the house together, and the doors close behind them.