In the picturesque town of Vevey, Switzerland, young Randolph C. Miller—a lively American boy—explores a hotel on his own. He approaches another American, Frederick Winterbourn, on the outdoor patio, promptly asking for a lump of sugar. The scene takes a turn when Annie P. Miller, affectionately known as Daisy, strolls in carrying a parasol, and Randolph enthusiastically introduces her as his older sister. Frederick quickly engages with Daisy, unearthing their shared origins from upstate New York and their anticipation of visiting Italy after their Swiss stay. Daisy, with her effervescent personality, overwhelms Frederick with her chatter, making plans to visit Chillon, a historic castle nearby.
Later, during a meal, Frederick's aunt, Mrs. Costello, warns him about the Millers, implying that Daisy and her mother exhibit bad manners through their familiarity with their European courier, Eugenio. She expresses concern that they may be socializing too freely, even dining with him. When Frederick shares his plans to visit Chillon with Daisy, Mrs. Costello scolds him for associating with what she deems an unrefined family. After dinner on the patio, Frederick finds Daisy there, and although she senses that Mrs. Costello disapproves of her, she reassures Frederick not to lose sleep over it.
As their connection deepens, Daisy pleads with Frederick to take her out on a boat. However, Mrs. Miller's disapproval prevails, and with Eugenio's intervention, Daisy is forbidden from going. As Daisy departs, she expresses a hope that her disappointment won't keep Frederick awake. The following morning, both Frederick and Daisy run to catch the ferry to Chillon, where Frederick reveals his past of schooling in Geneva and living in Europe. He recites a poem, stirring Daisy's desire for him to teach Randolph, but Frederick must decline the invitation to Italy, promising only to return to Geneva.
Frustrated by his rejection, Daisy confronts him outside, probing whether he has a romantic interest elsewhere; he adamantly denies it. Once their tension fades, she suggests he meet her when her family is in Rome, to which he eagerly agrees. Soon after, Frederick visits his aunt in Rome, whose gossip about Daisy reflects poorly—she has been seen mingling with an Italian gentleman. At a tea gathering hosted by Mrs. Walker, Frederick and Daisy's flirtation surfaces, leading Daisy to insist on escorting Frederick to her next outing.
However, complications arise when Mrs. Walker observes Daisy in the park with Mr. Giovanelli and feels compelled to intervene due to Daisy's perceived reckless association with two men. Despite Daisy’s pleas for Frederick's opinion on the matter, he aligns with Mrs. Walker's concerns. Frustrated, Daisy departs with Giovanelli while Mrs. Walker harshly insists Frederick join her. Fredericks waits for a moment but ultimately redirects his attention back to Daisy.
One evening, Mrs. Miller interrupts a party at Mrs. Walker's, explaining her daughter's lateness due to singing and playing piano with Giovanelli. As Daisy insists Giovanelli perform for the guests, Frederick warns her about the impropriety of her actions, stemming from different cultural expectations regarding flirtation. Daisy dismisses his advice and returns to Giovanelli after his performance. Mrs. Walker's disapproval reaches its peak when she announces that Daisy will never be welcome at her home again.
Frederick finds himself caught in the middle of a social quagmire as he repeatedly encounters Daisy and Giovanelli. At one point, Frederick witnesses Daisy flouting convention and behaving in ways that he believes are unbecoming. The tension escalates when Mrs. Miller expresses romantic expectations not entertained by Frederick, leading him to confront Daisy about her flirtations with Giovanelli, viewing her behavior as dangerously careless.
As the story unfolds, misfortunes befall Daisy when it's revealed she has contracted the Roman flu, compelling Frederick to visit the Millers' hotel suite. Mrs. Miller reveals that Giovanelli has mysteriously vanished from Daisy's side, and Daisy insists that Frederick know she is not engaged.
In an evocative turn of events, Frederick attends Daisy's funeral alongside Giovanelli and the Miller family. Reflecting on Daisy's innocence, Giovanelli explains that while she was like no other, their engagement was never a possibility. Ultimately, Frederick confides to his aunt his profound regret for the treatment of Daisy throughout their turbulent interaction, echoing a bittersweet awareness of his lost opportunity.
