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
Frederick Winterbourne meets Daisy Miller in Vevay and is tantalized and intrigued. They take a day trip alone together, which, back in the day, is a slightly risqué and exciting adventure. He looks forward to seeing her again in Rome.
Act II
Daisy Miller makes a scandal of herself in Rome by cavorting with a handsome Italian lawyer, Mr. Giovanelli. She is snubbed by Mrs. Walker, the doyenne of Roman expat society. Winterbourne, meanwhile, is torn between the disdain he shares with Mrs. Walker for Daisy's impropriety, and a curiosity and longing for Daisy and all that she represents.
Act III
Daisy contracts malaria while spending an evening alone with Giovanelli at the Colosseum. Winterbourne begins to realize that Daisy may have had eyes for him all along and was just using Mr. Giovanelli to occupy herself and make Winterbourne jealous. All of this knowledge comes too late: Daisy dies of malaria and Mr. Winterbourne goes back to his lackluster life in Geneva. Womp womp.