Bobby earns his living in Montmartre by duping naïve provincials, along with his pal Julien, as well as crooks and drug dealers, replaced by bicarbonate. His friend Monique, who dances at Le Paradou, is frightened by such a life. A crime of which he is not guilty occurs while he is robbing drug dealer Mureau. Bobby flees, but prefers to let himself be imprisoned. A serious accident is avoided. The brave Inspector Doirel is determined to set the boy straight. Monique will do her bit too.
