Agamemnon has been gone for ten years.
The Watchman reveals that Argos has been living without its king for ten years. He doesn't think that Clytemnestra, Agamemnon's wife, is managing things very well.
Agamemnon's coming home!
A signal fire announces that Agamemnon is coming home from the Trojan War. This raises the question of how he will be reintegrated into Argos, since so much has changed in his absence.
Agamemnon did some bad stuff back in the day.
The Chorus sings a song pointing out that Agamemnon sacrificed his own daughter, Iphigenia, to ensure good winds on the way to Troy. How is this going to play out when he comes back home and faces his wife, Clytemnestra?
Clytemnestra welcomes Agamemnon home.
Despite the uncomfortable history, Clytemnestra seems happy to see Agamemnon. But then she bullies him into walking over expensive purple fabrics, which Agamemnon considers disrespectful to the gods. The Chorus feels uneasy, but can't explain why.
Cassandra reveals the past and foretells the future.
Cassandra reveals the gruesome crimes that have taken place at the palace in the past generation, when Atreus (Agamemnon's father) butchered the children of his own brother Thyestes, and fed them to him. Cassandra also predicts that she and Agamemnon will soon be murdered, and hints that Clytemnestra will do it. The Chorus can't quite figure this one out.
Clytemnestra kills Agamemnon; Aegisthus reveals himself.
It turns out that Cassandra was right. Clytemnestra reveals that she has killed Cassandra and Agamemnon in revenge for Agamemnon's sacrificing Iphigenia, and also because she is carrying out the curse placed on the family in the past generation. Aegisthus, the son of Thyestes, reveals that he has taken part in the plot to avenge his siblings whom Atreus murdered. It also turns out that Clytemnestra and Aegisthus are now lovers.
Clytemnestra prevents Aegisthus from fighting the Chorus and leads him into the palace.
This conclusion is actually more like a cliffhanger. The Chorus wishes that Agamemnon and Clytemnestra's son Orestes will come back from exile to avenge his father. Clytemnestra says that she and Aegisthus will now become joint rulers in Argos.