The first words of the play are spoken by the Watchman sitting on the roof of the palace of Agamemnon in Argos, Greece. The thing is, Agamemnon isn't there. He's been gone for 10 years fighting the Trojan War. (For more info on the Trojan War, check out this website, or read all about it in our "Detailed Summary.") Then, just as the Watchman is telling us how much his life stinks, he sees a signal fire burning in the distance; this means that Troy has been captured. The Watchman goes off to alert Clytemnestra, Agamemnon's wife. In no time all of Argos is buzzing with activity, with many sacrifices being offered to the gods.
In the next scene, the Chorus, a collection of old men, gathers in front of the palace of Clytemnestra, Agamemnon's wife, who has been taking care of things while Agamemnon is gone. They want to know what all the fuss is about. Then they sing a song about the causes of the Trojan War and how, on his way there, Agamemnon sacrificed his and Clytemnestra's daughter Iphigenia to convince the goddess Artemis to send him good winds.
When the Chorus finishes its song, Clytemnestra appears outside the palace and tells them the good news of the war ending, and how she knows it. (She gives a long description of the chain of signal fires leading from Troy, in modern Turkey, to Argos.) Then she heads back inside the palace. The Chorus is still a bit skeptical about the news. Then, however, a Herald appears to announce that Agamemnon will be there soon. The Herald provides a convincing account of how Troy has been captured.
Then, sure enough, Agamemnon rolls up in his chariot. Beside him in the chariot is Cassandra, a Trojan princess whom he has taken prisoner. Clytemnestra appears outside the palace to welcome her husband. She commands slave-women to roll out a purple fabric for Agamemnon to walk on, so he won't have to touch the ground on his way from the chariot to the palace. Agamemnon is hesitant to do so – purple fabric was very expensive back then, and he is worried the gods will be offended if he, as a mortal, commits such an extravagant act. Eventually, however, Clytemnestra bullies him into obeying.
After Agamemnon walks over the fabrics into the palace, the Chorus members sing a song about how they are vaguely afraid, but don't know why. When they are done, Clytemnestra comes back out of the palace and tells Cassandra to come in. Cassandra doesn't obey, nor does she give any sign of having understood Clytemnestra's words. Eventually, Clytemnestra gives up trying to persuade her and heads into the palace.
This leaves the Chorus on stage with Cassandra. When the Chorus tries to persuade her to go into the palace, Cassandra, who has not opened her mouth so far, starts wailing horribly about how she is about to be killed. The Chorus starts questioning her about what she means. Cassandra, who has prophetic powers, reveals the past and the future. What she sees of the past is a gruesome crime committed by Agamemnon's father, Atreus. When Atreus was angry at his brother Thyestes for sleeping with his wife, he butchered Thyestes's children and fed them to him. This, Cassandra says, has brought a curse upon the house.
Next Cassandra looks into the future: she says that she and Agamemnon are about to be murdered; she drops hints that the murderer will be Clytemnestra, but the Chorus doesn't pick up on it. Finally, Cassandra says that she accepts her death, and heads inside the palace.
A short time afterward, the Chorus hears two cries coming from the palace: it is Agamemnon saying he has been killed. The Chorus members have a confused debate with one another; at first, some members urge immediate action, but they end up deciding to wait until they know all the facts.
The facts become clear soon enough, when the door of the palace opens, revealing Clytemnestra standing over the dead bodies of Agamemnon and Cassandra. Clytemnestra boasts that she killed Agamemnon to avenge their daughter Iphigenia. At the same time, she says that she is not responsible for the killing because she was just carrying out the curse that had been placed on the family by Thyestes, a generation ago.
Then (speak of the devil), who should appear on stage but Thyestes's son, Aegisthus! Aegisthus is now Clytemnestra's lover; it turns out that he plotted with her to kill Agamemnon in revenge for what Atreus (Agamemnon's father) did to his brothers and sisters. The Chorus is outraged by all this, and they are about to fight Aegisthus before Clytemnestra steps in. The play ends with the Chorus wishing for Agamemnon's son, Orestes, to come back from exile and avenge Agamemnon. Clytemnestra tells Aegisthus to ignore them; she leads him into the palace, saying that they will be joint rulers in Argos.