Agamemnon himself is the King of Argos, who is returning home from the The Trojan War after enslaving Cassandra, the daughter of the Trojan king, Priam. Cassandra is forced into being his concubine. Much to the distress of his wife, Clytemnestra, he finally returns home and she welcomes him back as if nothing is wrong, but she is unable to keep the bitterness out of her tone when she speaks of how long he had been gone. Stringing out extravagant tales of how much she missed him, in an attempt to make him feel uneasy and guilt stricken, she then orders maids to retrieve a purple cloth from his chariot, and spread it on the palace floor beneath his feet. But Agamemnon refuses to step on it.
When Clytemnestra speaks to him later she tries to convince him to walk on the purple carpet, comparing it to the sea. However, to her the carpet is symbolic. The 'Sea' she speaks of is the family feud, and the 'purple dye' is blood shed for revenge.* She continues to speak in thinly veiled metaphors of his impending fate, however Agamemnon never catches on. He finally re-enters the palace after much berating from Clytemnestra, when he is out of sight she then utters a terrifying cry of triumph that her plan is coming together. She quickly gives a prayer to Zeus so that her vendetta goes off without a hitch, and she follows him. She had been planning her husband's murder in the ten year long absence he was fighting in the war, during which she was having an affair with his cousin Aegisthus, who believes himself to be the rightful ruler. Clytemnestra is also vengeful against her husband for sacrificing their daughter Iphigenia.
Meanwhile we learn that Cassandra became possessed by the god Apollo, who gifted Cassandra with the power of clairvoyance so that she can foresee future events, after she promised to return his love. But when she broke her promise, Apollo cursed her so that no one who hears her prophesies will believe them until it's too late.
Cassandra has a vision of the walls of the palace dripping blood, Agamemnon's dead body and a sword in Clytemnestra's grasp. She also sees her own dead body lying still beside the king. She tries to warn the Elders of this horrible prophecy, but they are unable to understand what she's saying. Cassandra resigns to her fate, prays that she is given a quick death and enters the palace. Soon after the death-shriek of the king is heard; the Elders debate rapidly what to do; and the palace doors open to reveal Clytemnestra and the bleeding corpses of Cassandra and Agamemnon. Even though the Elders know that they should condemn Clytemnestra for her actions, the situation of her grief and suffering for years are making them unsure.
Aegisthus arrives to the scene and offers thanks to the gods. He is described by the Elders as a coward who refused to serve in the war, a lecher who seduced the king's wife in order to steal the throne back. However, he tells the horrifying story of what Agamemnon's father, Atreus, did to his father Thyestes, we realize that the same obligation which drove him to plot vengeance on the son of Atreus is exactly the same as that which now lies upon Orestes.
Aeschylus does not praise or excuse Aegisthus; but his insistence on presenting his case fairly ensures that the urgency of the central theme: What is justice? This is further heightened by the closing scene of the play.
Challenged by the Elders, Aegisthus makes a show of force; Clytemnestra pleads for restraint; and the Elders withdraw, shouting threats and defiance.
Agamemnon provides examples of:
- Asshole Victim: Agamemnon.
- Because Destiny Says So: Agamemnon sacrificed Iphigenia because of this, and Cassandra rushes to her death at the hands of Clytemnestra since it's useless to delay it.
- Big, Screwed-Up Family: The fourth and fifth of five messed-up generations.
- Bystander Syndrome: Agamemnon cries for help from inside the palace (i.e. off-screen). The Chorus reacts thus.
- The Cassandra: Literally.
- Cassandra Truth: The original one, since it comes from the Trope Namer.
- Death of the Old Gods: The Chorus describes how Ouranos was defeated by Kronos, and Kronos in turn was defeated by Zeus.
- Due to the Dead: It's a big deal that the bodies of some Greek soldiers were left in Troy without a proper burial.
- Doublespeak: Many of Clytemnestra's speeches have double meanings.
- Downer Ending: It ends with both Agamemnon and Cassandra dead.
- Face Death with Dignity: What Cassandra decides to do, since it was foreseen anyway.
- Foreshadowing: Mainly done by the Chorus and Cassandra. Not only does she foresee Agamemnon's and her own demise, she also predicts the events of The Libation Bearers which is Orestes' vengeance upon Clytemnestra.
- Greek Chorus: This is basically the role of the Eldest of Argos in the story.
- God Save Us from the Queen!: Clytemnestra, though she's not explicitly a bad ruler, the Elders just don't like her because she's too crafty for their liking. And, well, a woman.
- He Who Fights Monsters: Two in the backstory: Thyestes, who was tricked by his brother Atreus into eating his own sons, has raped his own daughter in order to raise a son who would kill Atreus. Furthermore, his son/grandson Aegisthos was raised as a weapon of vengeance, so it's not surprising he became an unpleasant character himself.
- Historical Fantasy: Archaeologists have found there was a real Agamemnon. Whether he was killed by his cousin and avenged by his children (as well as whether Atreus was as evil as depicted in Classical Mythology or subjected to a Historical Villain Upgrade) cannot be known.
- Hostile Weather: The Greek fleet is plagued by storms.
- I Will Wait for You: What Agamemnon expected from Clytemnestra. She didn't.
- Keep the Home Fires Burning: The play as a whole shows how this can go wrong, since after being left at home Clytemnestra was left to wallow in her hatred of her husband and had time to plot his murder and take his cousin as her lover.
- Misblamed: A footnote in one translation notes that the Chorus does this to Helen, claiming that she was ultimately responsible for the war.
- Mr. Exposition: The Watchman and Cassandra herself as she starts narrating the story of Agamemnon's ancestors.
- Offing the Offspring: Agamemnon's sacrifice of his daughter Iphigenia before he left for Troy is part of the reason Clytemnestra plots his murder, especially since he tricked Clytemnestra into sending their daughter to him under the pretext of marrying her to Achilles and only revealed his true intentions once it was too late.
- The Ophelia: Subverted. The Elders think Cassandra is one of these and treat her as a madwoman at first, but then she starts talking about the Royally Screwed Up story of Argos as if she had been there...
- Noble Bird of Prey: Bird symbolism is used throughout the play.
- Pride: Agamemnon. Clytemnestra encourages him to commit hubris, but he could have said no.
- Psychic Powers: Cassandra's clairvoyance, given to her by Apollo.
- Royally Screwed Up: The Atreides were the poster guys of this trope in Classical Mythology. All the madness started with Tantalus serving his own son to the gods to prove they're not omniscient, continued with the feud between brothers Atreus and Thyestes, and know the feud continues with their respective sons Agamemnon and Aegisthus.
- Secondary Character Title: Despite being the character of the title, Agamemnon isn't given much action (aside from his murder) and the true protagonists is Clytemnestra.
- Second-Hand Storytelling: The Greek Chorus and Cassandra are the main source of the antecedents.
- Straw Feminist: A possible interpretation of Clytemnestra's character.
- Skepticism Failure: Happens to everyone who hears Cassandra's claims (not by their own fault though, but because of the curse Apollo placed on her).
- Soundtrack Dissonance: According to Aristophanes. "Phlattothrattophlattothrat..."
- Virgin Sacrifice: Agamemnon kills one of his daughters, Iphigenia, for a favorable wind in order to go to war.
- You Can't Fight Fate: Clytemnestra's justification for her actions. Once Agamemnon killed their daughter, he sealed his own fate.
- Your Cheating Heart: Clytemnestra cheats on Agamemnon with his cousin Aegisthus, while Agamemnon himself has taken Cassandra as his concubine, but guess which adulterer is treated less sympathetically.