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
Mrs. Alving wants to bury her husband and all the lies she told about him, but there's a hitch. Captain Alving's illegitimate daughter – living proof of his true nature – has attracted the sexual attention of his son.
Act II
Mrs. Alving is torn between telling her son the truth and preserving a fatherly ideal for him. She figures out how to do both, but is interrupted by the Orphanage burning down.
Act III
Mrs. Alving describes Captain Alving's life as it was, breaking the silence of twenty years. Everyone else leaves, and Oswald is left alone with his mother. He asks her to put him to death in the event that he declines (he's very sick), and she agrees, believing it will never happen. The play ends with the attack he feared. Mrs. Alving stands holding the morphine, undecided.