Daisy Miller opens in Vevey, Switzerland, where a young expatriate American named Winterbourne is visiting his aunt. He has lived in Geneva for many years and identifies greatly with his European peers. When the flirtatious New Yorker, Daisy Miller, arrives on the scene, she catches his eye. Even for the relaxed resort environment of Vevey, Daisy's behavior is extreme because she speaks to strange men without the presence of an escort, and she brags about her many male suitors. Winterbourne is entranced by both Daisy's beauty and her coquettish behavior, and even though his aunt warns him that the Millers are of low class in spite of their money and he is determined to spend more time with her. They go sightseeing at the nearby Chillon Castle, where Daisy is more interested in Winterbourne than in exploring the ruins. She demands that he travel with her family, and when he tells her he cannot, she becomes jealous and accuses him of going back to Geneva for another woman.

Winterbourne arrives in Rome in winter, again visiting his aunt, and meets Daisy and her family at the home of Mrs. Walker, a mutual American friend. Daisy again plays the coquette, though in Rome the reaction to her flirtatiousness is much stronger than it was in Vevey. She makes friends with a shady local, Mr. Giovanelli, with whom she scandalously strolls around in public and rumors circulate about her character. Mrs. Walker tries to reason with her but fails, and ultimately shuns Daisy. When Winterbourne attempts to warn Mrs. Miller about her daughter's behavior, she tells him she believes Daisy to be engaged to Giovanelli. Eventually their paths stop crossing, but Winterbourne keeps hearing rumors of the many men who call at Daisy's hotel. One night he takes a midnight stroll near the Colosseum and sees Daisy there alone with Giovanelli. He decides she is not a "nice girl" after all and loses interest in her. Daisy takes ill with malaria, but it is not until after her death that Winterbourne learns she was never engaged to Giovanelli; and was, in fact, innocent of all the things of which she had been accused.