Sylvie is a young cashier in a Montmartre café, "le bar des Philosophes", owned and run by Monsieur Marcel, a former safe cracker. One day, the pretty creature accepts to give a helping hand to two regulars, Francis and Josélito. But what the naive girl does not know is that the two men are gangsters and that they plan to steal 50 million francs deposited in the safe of Maître Roland Devert, a charming young notary. Actually things do not go according to plan all the more as a love story between the notary and Sylvie was not at all on the two gangsters' agenda.