A modest professor of philosophy plunges in spite of himself into the world of the underworld, when he goes to Paris, where he must attend a congress.

Bloody and fast hold-up near the oil refineries of the Etang de Berre. Pursued by a rival gang, A gangster, Toni, fled with a satchel containing a very large sum of money (thirty million francs). On the train that takes him to Paris, Toni threatens with his gun Justin Mignonnet, an honest professor of philosophy leaving Arles to attend a congress, and forces him to put this bag, the contents of which he ignores, in his suitcase, and makes him promise to bring it back to his men the next day at the Pigalle hotel, located in rue Fontaine - Completely lost, Mignonnet decides to obey Toni's orders. At the time of the restitution, it is a young woman who presents herself, because another gang seeks to seize the loot. They meet at the hotel where she pretends to be her niece and must, from this moment, be wary of everyone. Little by little, the underworld of the capital takes him for a real boss. Taking advantage of a moment of respite from his opponents, he will take the money to charity before resuming his train for Arles.