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.
Orestes comes home and links up with his sister and the Chorus.
With a little help from his friends, Orestes enters the palace in disguise.
Orestes kills Aegisthus and Clytemnestra, then goes crazy.