A repentant gambler, Simon is a senior inspector in the banditry repression brigade headed by his friend Pierre Tramoni. Pierre is brilliant and wants to go far. Simon has no ambition and only one passion: gambling. For several months, Simon has been trying to catch the Djian brothers, young and dangerous bank robbers, in the act. He fails but discovers, through an informer, strange links between Pierre and the Djians, during his investigation which leads him back to the gambling halls. When the Djians finally fall, the treachery of Commissioner Tramoni is revealed.

L'inspecteur Simon n'aime rien tant que d'arpenter le bitume et se frotter au milieu. Il rêve de prendre en "flag" (flagrant délit) un dangereux gang de braqueurs. Devant les réticences de son supérieur, l'ambitieux commissaire Pierre Tramoni, à lui donner les moyens pour y parvenir, Simon s'interroge. Quels rapports le carriériste Tramoni entretient-il avec la pègre parisienne ?