This is the first of a cycle of three plays written by the Greek playwright Aeschylus. When the play begins, King Agamemnon is still away at the Trojan War. His wife Clytemnestra (kly-tem-NEST-ra) and his young children, Orestes (a boy) and Electra (a girl) are at home in Mycenae. But Clytemnestra is very angry at Agamemnon for killing their daughter Iphigeneia, and she has been letting a cousin of Agamemnon's named Aegisthus rule the kingdom while he was away, instead of keeping it safe for her husband.
When Agamemnon gets home, he acts very arrogant. He does not pay the gods the respect that they deserve. For instance, he walks on a red carpet to the door of his house, even though this should be sacred to the gods. This is hubris, and the gods punish it. As soon as Agamemnon gets inside the house, Clytemnestra and Aegisthus murder him (off-stage; in Greek plays the action usually takes place off-stage).
To find out more about this play, check out these books from Amazon.com or your library:
Greek Theatre, by Stewart Ross (1999). For kids.
Greek and Roman Theater, by Don Nardo. For teenagers.
The Oresteia, by Aeschylus, translated by Robert Fagles (Penguin Classics). The most famous of the plays Aeschylus wrote. Fagles is a great translator! Includes a version for performance.
Aeschylus, by John Herington (1986). A discussion by a specialist about the life of Aeschylus and why his plays are written the way they are.
Greek Tragedy: A Literary Study, by H. D. F. Kitto (reprinted 2002). A classic discussion of the meaning of Greek tragic plays, by a specialist.