Marguerite discovers that Adolphe has syphilis, Jouin identifies the body in the Bois de Boulogne, and Cochefert reveals that the dead man is linked to an unsolved murder.

Armed with information provided by Dr Socquet, Cochefert makes an eventful visit to Dr Verlot. Lépine asks his daughter to invite L'abbé Gardey to supper to provide moral support, and Jouin and Fiersi join forces to break into the Château de Bagatelle.