Reinette and Mirabelle are two young girls. Reinette lives in the countryside, Mirabelle in Paris. They meet during a holiday of Mirabelle's in the country when Reinette helps her to repair the tube of her bicycle and shows her the beauties of nature and in particular the 'blue hour'. They like each other and decide to take a flat together in Paris, where they'll attend the University. But it isn't so easy to live together when they're so different: as Reinette is simple and enthusiastic, and Mirabelle is obscure and lazy.