London. Diana is a young model; the older Robert is a literary interviewer/director for television arts programmes. They meet by chance when Diana is spotted on the street by Robert's roving film crew and interviewed by him about young people's views on convention. Diana is invited to watch the final edit in the TV studio and there their relationship starts. After liaisons in bleak hotel rooms they leave their partners (and, in Robert's case, children) and move into an apartment.
As a couple, they become part of the fashionable London media/arts set. Initially, Diana is jealous when Robert sees his wife while visiting his children, but she quickly loses this attachment when she mixes with the predatory males of the media, arts and advertising scene, particularly Miles, a powerful advertising executive for the "Glass Corporation" who gets her a part in a trashy thriller after she has sex with him. The bookish Robert prefers the quiet life; it is he who now becomes jealous, but increasingly detached, depressed and lonely.
Diana attends a high-class charity draw for world hunger for which she is the face. The event, adorned by giant images of African famine victims, is at the height of cynical hypocrisy and bad taste, showing Diana's rich white set, which now includes the establishment, playing at concern, gorging themselves, gambling and generally behaving decadently.
Already showing signs of stress from constantly maintaining the carefree look demanded by the false, empty lifestyle to which she has become a prisoner, Diana becomes pregnant, and has an abortion.
She flies to Paris with Miles for more jet-set sophistication. She finds the wild party, beat music, strip dance mind game, cross dressing and predatory males and females vaguely repellent and intimidating, but Diana holds her own, gaining the respect of the weird crowd when she taunts Miles in the game. On her return to London, Robert calls her a whore and leaves her, for which she is not emotionally prepared. Ironically, Miles casts her as "The Happiness Girl" in the Glass Corporation's advertising campaign for a chocolate firm.
On location at a palazzo near Rome, Diana smiles in her medieval/Renaissance costume and completes "The Happiness Girl" shoot. She is much taken with the beauty of the building and the landscape and gets on well with the Prince, Cesare, who owns the palazzo, and his eldest son, who is her age. Cesare's wife is dead. With the gay photographer who has created her now famous look and who is the only person who has shown her any real understanding and friendship, Diana decides to stay on in Italy. They stay in a simple house by a small harbour in Capri. Diana flirts half-heartedly with Catholicism. They are visited by Cesare, who arrives in a huge launch, invites them on board and proposes to Diana. Diana politely declines, but Cesare leaves the offer open.
Diana returns to London and goes to a refreshingly innocent disco with Miles. Robert has aged. Soon disillusioned with Miles and the vacuous London jet set, Diana flirts with the Catholic Church again. Impulsively, she flies out to Italy and marries the Prince, which proves to be ill-considered. Though waited on hand and foot by servants, she is almost immediately abandoned in the vast palazzo by Cesare, who has gone to Rome, presumably to visit a mistress.
Diana flees to London to Robert, who, taking advantage of her emotional vulnerability, charms her into bed and into what she thinks is a stable long-term relationship. In the morning, in self-disgust, he tells her that he's leaving her and that he fooled her only as an act of revenge. He reserves a flight to Rome, packs her into his car and takes her to London airport to send her back to her life as the Princess Della Romita. At the airport, Diana is hounded by the press, who address her reverentially as Princess. She boards the plane to leave.
