The play is set sometime in the 12th or 13th centuries BCE, and begins in Argos, Greece. A Watchman, stationed on top of the palace, waits for a signal fire indicating that the ten-year Trojan War (between the Greeks and the Trojans) has come to an end. He sees the fire light and rushes out to tell queen Clytemnestra the good news.
The Chorus, a group of elderly men too old to have fought in the war, come out and explain that the Trojan War began out of a feud between Paris, the Trojan prince, and Menelaus, the Greek king. Menelaus is the brother of Agamemnon, who is Clytemnestra’s husband and the king of Argos. Paris stole Menelaus’ wife Helen, and as a result Menelaus and Agamemnon led a fleet to Troy to avenge the insult. The Chorus also tells us that in order to persuade the goddess Artemis to allow his fleet to pass, Agamemnon sacrificed his and Clytemnestra’s daughter, Iphigenia.
Clytemnestra enters and the Chorus asks why fires of sacrifice are being lit around the palace. Clytemnestra tells the Chorus that Troy has fallen, but they remain skeptical, so she explains the system of signal fires that allowed the news to travel so quickly. The Chorus then thanks the gods and muses on the gods’ tendency to punish mortals who are prideful. A Herald enters and confirms that Troy has indeed fallen, and recounts some of the hardships of the war. Clytemnestra chides the Chorus for being skeptical but the Herald admits that not all the news is good – a storm has separated Agamemnon and Menelaus’ fleets, and Menelaus is missing.
The Chorus details how Helen incited the fall of Troy. Finally the Chorus welcomes Agamemnon, who enters riding in a chariot with Cassandra, Paris’ sister—a prophet of the god Apollo, and Agamemnon’s new slave. Clytemnestra meets Agamemnon outside the palace and implores him to enter into the palace walking on a carpet of purple tapestries. After his victory in Troy, Agamemnon is reluctant to do anything that may be seen as an act of defiance to the gods (as this might be), but Clytemnestra convinces him to walk on the tapestries, and he does so.
The Chorus suggests an impending sense of dread for what’s to come. Clytemnestra tries to force Cassandra to go into the palace, but Cassandra remains silent, and Clytemnestra gives up and leaves her in the chariot. Cassandra, possessed by the god Apollo, begins to cry out, and her thoughts eventually form a prophecy in which she predicts Agamemnon’s murder in detail, as well as her own death. She enters into the palace to meet her fate.
All of a sudden, we hear Agamemnon from within the palace screaming that he is being attacked. The Chorus breaks out into chaos, unsure of how to respond. A blood-soaked Clytemnestra appears, and the palace doors open to reveal the dead bodies of Agamemnon and Cassandra. Clytemnestra explains that she has committed the murders to avenge her daughter Iphigenia’s sacrifice. Aegisthus, Clytemnestra’s lover and Agamemnon’s cousin, joins Clytemnestra and explains that the revenge is twofold—long ago, Agamemnon’s father Atreus had killed and cooked Aegisthus’ brothers and fed them to their father, Thyestes. As Aegisthus’ soldiers surround the Chorus, the Chorus prays that Agamemnon’s son Orestes will return to Argos to set things right.