The novel centers around Milly Theale, an American heiress who becomes part of European high society in Rome. She is invited to stay with the Roccanes, a prominent Italian family. However, her stay is complicated by her cousin, Signor Herbert, who arrives with his own secrets and social ambitions. The narrative alternates between Milly's perspective and that of Olive, her American cousin, who observes Milly's experiences from a more detached viewpoint. The story explores Milly's relationships with the Roccanes, particularly the enigmatic Signora Titti, and her interactions with Signor Herbert, whose true intentions remain ambiguous. James masterfully portrays the opulent but often hollow world of European aristocracy and contrasts it with the more direct, albeit sometimes less refined, American pragmatism.