Not surprisingly, this didn't sit well with his wife, Clytaemnestra. While Aggie was out of town, she has taken up with Aegisthus, who also has a pretty awful back story. (Before his birth, his brother was killed and put into a stew. Aegisthus himself, was born of spiteful incest and set to kill his own father.) Clyt and Aegis both lay in wait to kill Agamemnon as soon as he got home. So arrive, he does, and he is whisked off by the servants.
We then meet Cassandra, pretty much the only sympathetic figure in the whole story. She was taken as a slave from Troy and is now Agamemnon's concubine. She suffers a unique curse. Cassandra can see the future, but she is doomed to be ignored. In this case, she sees her own death and yes, she's right again.
The action happens offstage. Clytaemnestra kills both Agamemnon and Cassandra. Aegisthus dances on the bodies. The chorus, made up of the men of Thebes, tells him how awful he is and tell him that he'll get his end.
Cassandra is the high point of the play. I can easily imagine that she gave the Greeks chills!
the pain, the terror! the birth-pang of the seer
who tells the truth-
it whirls me, oh,
the storm comes again, the crashing chords!
Look, you see them nestling at the threshold?
Young, young in the darkness like a dream,
like children really, yes, and their loved ones
brought them down . . .
their hands, they fill their hands
with their own flesh, they are serving it like food,
holding out their entrails . . . now it's clear,
I can see the armfuls of compassion, see the father
reach to taste and-
For so much suffering,
I tell you, someone plots of revenge.