At certain parts of the novella, especially in the first part, the mood remains mostly light and playful. This mood can be found in the narrator's sarcastic, witty introduction to Winterbourne. Other examples include scenes in which Winterbourne interacts with the young and brazen Randolph, as well as the quippy dialogue between Winterbourne and Daisy when they first get to know each other.
In the second part of the novella, the mood increasingly goes through fluctuations. Rome seems to make Winterbourne more pensive and gloomy than he was earlier in the narrative. He and Daisy continue to enjoy their playful rapport, but the presence of Giovanelli and the scrutiny of the American community—as represented by the serious Mrs. Walker—get in the way of genuine connection. Whereas Vevay is marked by a charming, lively, fresh atmosphere, the narrator describes Rome with heavier, grimmer diction. Rome is certainly also beautiful, but in a more severe, less hospitable way. This directly shapes the mood.
The gradual shift in the overarching mood is not helped by Winterbourne's exhaustion at never getting any further in his pursuit of Daisy, after months of feeling attached to her. Rather than feeling delighted by her refreshingly spontaneous and elusive behavior, as he does in the early part of their relationship, he increasingly feels frustrated by it.