Paris, 24th December 1904. An unidentified body is found in the Bois de Boulogne gardens. Inspector Antoine Jouin leads the inquiry, which takes him into clandestine territory – the woods where the Parisian elite go to satisfy their darkest desires…

Lépine asks Jouin to remain discreet in his report of a death to avoid a scandal. Marguerite Steinheil prepares for the opening of a new collection of her husband's work, to which she has invited Lépine.

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.

Louise Lépine finds an unlikely ally in her attempts to persuade her father to postpone his retirement, Jouin hear testimony from the mother of Paul Cetinet and Fiersi, now a free agent, talks to one of the maids at Château de Bagatelle.

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

Guichard falls into the trap when he is lured to Château de Bagatelle by a phone call from Fiersi's son. And Commissioner Lépine confronts Fondari, the head of the vice squad.