A quick response and more later.
Berkoff adapted the work from Aeschylus and in its time it was a well known story. Both Berkoff and Aeschylus deal with revenge as a major theme, but revenge that resolves nothing.
The first scene in Berkoff’s production is not in Aeschylus but gives the origin of the curse on the House of Atreus. Thyestes and Atreus are enemies, but Atreus invites Thyestes for a meal.. Berkoff describes the meal, a vile affair with everyone looking at everything Thyestes eats. Then Atreus tells Thyestes he has eaten his own sons who Atreus had murdered then cooked. Thyestes curses the House of Atreus.
Atreus has two sons, Agamemnon and Menelaus. Thyestes has another son Aegisthus. The story moves to the next generation.
(the image comes from here
http://en.wikipedia.org/wiki/Atreus )
Agamemnon marries Clytemnestra and they have a daughter Iphigenia. Menelaus marries Helen. Helen is kidnapped by The Trojans so Agamemnon wants to sail to Troy to fight the Trojans. But there is no wind for the ships and he discovers he must sacrifice his daughter before the gods will allow the ships to sail. He sacrifices Iphigenia and sails for Troy. The curse of Atreus continues.
The main revenge of the play is that of his wife Clytemnestra who wants Agamemnon to suffer for killing their daughter.