Daisy Miller begins in the resort town of Vevay, in Switzerland, where a young expatriate American, Mr. Winterbourne, has arrived from Geneva (where, according to various rumors, he either studies or pursues an older foreign lady) to spend some time with his aunt, Mrs. Costello. Winterbourne encounters a young boy, also American, who is talkative and immediately engages Winterbourne in conversation. The boy’s sister arrives, and Winterbourne learns their names: Daisy and Randolph Miller. Winterbourne finds Daisy quite pretty, and though she initially pays him little attention, she soon warms to him and starts chatting away, astounding him with her talkativeness and her lack of the shy feminine attitude he has grown used to in Geneva. Daisy asks if he might take her to the Château de Chillon, a castle across the lake, which she’s wanted to see, and he agrees.
Winterbourne asks his aunt about the Millers, and she tells them that they are vulgar people. This disappoints him, as he is fascinated by Daisy and wanted to introduce her to his aunt. Mrs. Costello is particularly offended that Daisy plans to go with Winterbourne, unaccompanied by anyone else, to the chateau. That evening, Winterbourne meets Daisy again and, later, her mother, Mrs. Miller, who seems alternately bemused and unaffected by Daisy’s up-front attitude towards Winterbourne. Daisy asks Winterbourne to take her out on a boat alone, even though it is late at night, but the courier, Eugenio, arrives to announce that he’s finally gotten Randolph to go to bed, and they all retire.
A few days later Winterbourne takes Daisy to the chateau. He enjoys watching her and listening to her chatter on, but he’s a little disappointed that, although she is quite frank with him, she doesn’t seem nervous as he imagines a young lady would in such a situation if she were actually attracted to him. But when Winterbourne reveals that he has to return to Geneva shortly, Daisy is indignant, and she imagines he has a lover there. She continues to complain until Winterbourne agrees to meet her in Rome the following winter, where his aunt has an apartment, and where the Millers are going next.
Winterbourne arrives in Rome to learn, from his aunt, that Daisy is causing a great deal of talk in society—she is wandering around alone and always seems to be surrounded by admiring Italian gentlemen. Somewhat put off by the idea of such competition, Winterbourne goes to see a friend from Geneva, Mrs. Walker. There, however, he encounters the Millers, who are also paying Mrs. Walker a visit. Winterbourne talks to Mrs. Miller for a while, before greeting Daisy, who is quick to vocalize her dissatisfaction with the way he left her at Vevay. Mrs. Walker, however, finds it quite strange that she would act in such a way, since Daisy and Winterbourne had only known each other for a few days. Daisy announces that she is planning on going for a walk, to meet her Italian friend, Mr. Giovanelli. Mrs. Walker begs her not to go alone, and Daisy cheerily suggests that Winterbourne accompany her. They meet Giovanelli, a dashing Italian man, in the Pincian gardens, to walk. Before long, however, Mrs. Walker pursues them in a carriage, and she begs Daisy to get inside, since everyone is talking about her. Daisy holds her ground and refuses. Mrs. Walker, angry and frustrated, then insists that Winterbourne accompany her, and he leaves Daisy and Giovanelli. It begins to become difficult for Winterbourne to call on Daisy, as she is always out or with Giovanelli. At Mrs. Walker’s party, Mrs. Miller comes alone, saying that Daisy has gotten wrapped up in playing piano with Giovanelli, and will come later. Mrs. Walker is shocked and indignant, and decides to refuse to talk to Daisy. Daisy comes with Giovanelli and is bright and cheerful as usual. When Winterbourne tries to warn her about what people are saying, Daisy says he’s being disagreeable, and she spends the rest of the night with her Italian friend. When she goes to leave, Mrs. Walker turns her back to Daisy, and Winterbourne watches, feeling sorry for her, as Daisy seems confused and uncertain.
Mrs. Walker and her friends, after that, stop inviting Daisy to their parties. One day Winterbourne encounters Daisy with Giovanelli at a tourist site. Again he tries to tell her what people are saying, and she says that he should prevent people from being unkind. Winterbourne asks if she’s engaged to Giovanelli, and Daisy first says she is, then—when Winterbourne says he believes her—that she’s not.
A week later, Winterbourne eats out and decides to stroll home through the moonlight. He peeks into the Coliseum, which is beautifully lit up, but then remembers that this is how one contracts the Roman fever. He is about to leave when he sees Daisy and Giovanelli there. Winterbourne feels both bitter and relieved, now that he thinks he finally understands that Daisy is simply a low, disreputable woman. Still, it’s dangerous for her to be there at this hour, and when he asks why the Italian has brought her, Giovanelli says he can’t prevent Daisy from acting as she wishes. Finally Winterbourne convinces them to leave. But soon after, Daisy falls seriously ill. When Winterbourne calls at their hotel, Mrs. Miller tells him that Daisy wanted him to know that she wasn’t engaged to Giovanelli—she was very insistent about it. Daisy dies of the Roman fever soon after, and is buried in Rome. Not long after that Winterbourne returns to Geneva, but he comes back to Vevay to visit his aunt the next year, where he reflects that he misunderstood Daisy, and that she would have appreciated his esteem. The book ends with Winterbourne back in Geneva, either studying or, as others say, pursuing a foreign lady.