Young Monique leaves the preventorium where Doctor Delaunay has lovingly cared for her. A painful return. Her mother Denise lives with Eugène, a handsome, disreputable young man who's easy to please and, on occasion, won't hesitate to steal. He hangs around Monique, excited by her freshness, until the day he is shot on suspicion of the disappearance of a mobster. Monique is reunited with Dr Delaunay, much to the satisfaction of her guardian Gilbert, known as Patte-en-moins, owner of the local bistro.
Tongues start to wag after a famous Parisian music hall dancer is rushed to hospital after being found poisoned by arsenic at the Concert Mayol.
Jean-Louis, Danielle's fiancé, a nurse, becomes blind following an explosion in the mine where he works. Coming from Lyon, Doctor Coudray, whose natural son Gilbert - who loves Danielle - dies while helping his father threatened by the person responsible for Jean-Louis' accident, will work to restore his sight.
Sherlock Coco, famous detective, tries to thwart the machiavellian plans of the trio of shock. Croquignol, Ribouldingue and Filochard must be extra vigilant, but the profit motive is too strong. As a corollary, the famous pink diamond is also of interest to their legendary enemy, Jo Papillon. But always inventive, they will work out multiple scams and tricks to reach their goals.