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
Part I: Flaubert has actually divided his novel into three appropriate sections for us. This first one ends as Emma’s depression and discontentment motivates the move from Tostes.
Act II
Part II: Emma begins to wonder about experimenting with life outside marriage; she falls in love with Léon, but actually embarks upon an adulterous affair with Rodolphe. This "act" finishes with her reunion with Léon.
Act III
Part III: Emma starts her affair with Léon, but things quickly spin out of control, both emotionally and financially, and end in ruin.