For a three-act plot analysis, put on your screenwriter’s hat. Moviemakers know the formula well: at the end of Act One, the main character is drawn in completely to a conflict. During Act Two, she is farthest away from her goals. At the end of Act Three, the story is resolved.
Act I
Orestes comes home and links up with his sister and the Chorus.
Act II
With a little help from his friends, Orestes enters the palace in disguise.
Act III
Orestes kills Aegisthus and Clytemnestra, then goes crazy.