At a hotel in the resort town of Vevey, Switzerland, a young American named Winterbourne meets a rich, pretty American girl named Daisy Miller, who is traveling around Europe with her mother and her younger brother, Randolph. Winterbourne, who has lived in Geneva most of his life, is both charmed and mystified by Daisy, who is less proper than the European girls he has encountered. She seems wonderfully spontaneous, if a little crass and “uncultivated.” Despite the fact that Mrs. Costello, his aunt, strongly disapproves of the Millers and flatly refuses to be introduced to Daisy, Winterbourne spends time with Daisy at Vevey and even accompanies her, unchaperoned, to Chillon Castle, a famous local tourist attraction.
The following winter, Winterbourne goes to Rome, knowing Daisy will be there, and is distressed to learn from his aunt that she has taken up with a number of well-known fortune hunters and become the talk of the town. She has one suitor in particular, a handsome Italian named Mr. Giovanelli, of uncertain background, whose conduct with Daisy mystifies Winterbourne and scandalizes the American community in Rome. Among those scandalized is Mrs. Walker, who is at the center of Rome’s fashionable society.
Both Mrs. Walker and Winterbourne attempt to warn Daisy about the effect her behavior is having on her reputation, but she refuses to listen. As Daisy spends increasingly more time with Mr. Giovanelli, Winterbourne begins to have doubts about her character and how to interpret her behavior. He also becomes uncertain about the nature of Daisy’s relationship with Mr. Giovanelli. Sometimes Daisy tells him they are engaged, and other times she tells him they are not.
One night, on his way home from a dinner party, Winterbourne passes the Coliseum and decides to look at it by moonlight, braving the bad night air that is known to cause “Roman fever,” which is malaria. He finds Daisy and Mr. Giovanelli there and immediately comes to the conclusion that she is too lacking in self-respect to bother about. Winterbourne is still concerned for Daisy’s health, however, and he reproaches Giovanelli and urges him to get her safely home.
A few days later, Daisy becomes gravely ill, and she dies soon after. Before dying, she gives her mother a message to pass on to Winterbourne that indicates that she cared what he thought about her after all. At the time, he does not understand it, but a year later, still thinking about Daisy, he tells his aunt that he made a great mistake and has lived in Europe too long. Nevertheless, he returns to Geneva and his former life.