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
Nel befriends Sula and they grow into adulthood together...when Sula has an affair with Nel's husband.
Act II
Nel and Sula see each other for the first time in three years. Sula fails to apologize or offer any real explanation for the affair.
Act III
Sula dies without having mended her relationship with Nel. It's not until after Sula is gone that Nel realizes how much she misses her. She visits Sula's grave and finally really mourns for her.