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.
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.
Part III: Emma starts her affair with Léon, but things quickly spin out of control, both emotionally and financially, and end in ruin.