Desideria, an obese and unhappy girl, lives with her widowed mother and rich Viola who, one day, reveals to her that she is not her real mother and that she bought her from a prostitute. The night before, Desideria also happens to witness a scene: the mother is sodomized by the administrator of the Tiberi family assets under the gaze of the housekeeper Chantal. These events lead to a change in the life of Desideria who, growing up, becomes a slender and beautiful teenager who unleashes in Viola a morbid attraction. Desideria, as an act of rebellion against his adoptive mother, decides to imitate street prostitutes, having as his first and only client Erostrato Occhipinti, a broke-up Sicilian, which makes Desideria believe that he is an active member of extreme left-wing groups that organize thefts and kidnappings to finance the imminent revolutionary overthrow of the bourgeois order, even if in fact he has in the past also carried out activities as an informant for the police. The girl plans to use Erostrato to kidnap Viola, so as to extort a ransom through Tiberi. Erostrato does not take his idea seriously, but nevertheless tells her about Quinto, a fellow Milanese arriving in Rome who is able to authorize and organize the kidnapping.
