Aeschylus, Orestia ["Plays About Orestes"] (before 456 B.C.E.)
The Greeks took for granted knowledge of the general story of the Trojan War, its causes and consequences, which they believed to involve their ancestors in a titanic struggle which the gods, themselves, both provoked and aided. If you are not familiar with the general story, you may want to look at a quick summary of events in Homer's Iliad and Odyssey, the Bronze Age (c. 9th-7th century B.C.E.) epic poems which the Athenians used as the cornerstone of their educational system. There also are some concepts which are peculiar to Classical Greek culture which may help you understand better the dynamics of this dramatic trilogy's story of revenge and restoration of order.
The Greek gods and goddesses were central to the Greek understanding of how the universe functioned. Just as modern physics tells us of "neutrinos" and "the weak force" that are invisible entities and powers which structure matter and energy, the Greeks understood the movement of the seas and planets, the seasons, catastrophes and ordinary daily events as products of divine agents. (To the degree that we don't understand what physicists are telling us, we're not much better off than the Greeks when it comes to actually understanding what the universe is made of!) If you are not familiar with the Greek gods and goddesses, please consult the "Greek Gods" page for short description and consider printing it out for our reading of Aeschylus.
After the gods, family connections were thought most important in determining human character and fate. Ancestral spirits were worshiped and consulted for advice, and ancestral virtues and vices were thought to have a hand in creating contemporary events. In all Greek literature, perhaps no family was quite so marked out as an example of this as the House of Atreus. Here's the background to Agamemnon's homecoming and the events which follow.
Geneology of the House of Atreus
Tantalus (killed son, Pelops and tried to feed body to gods--
* punished in Underworld
(seduced Atreus' wife)--Thyestes Atreus--(in revenge, murdered all Thyestes' sons except Aegisthis)
* * *
Aegisthis & 3 other sons Agamemnon Menelaus
|>seduces Clytemnestra |---->[marries Clytemnestra [marries Helen
Orestes Electra [Iphegenia]
|->(kills Klytemnestra and Aegisthis with Electra's help)