Jacques Arnaud arrives in a small town somewhere in the province. Soon a citizen reports to him that strangers have broken into his house where they stole a mysterious "black dossier". This file had been given to him by a recently demised citizen named Le Guen. The content of this file included information on the town's foremost businessman who behind his proper façade seems to be a ruthless fraudster. Arnaud has the corps of Le Guen exhumed and proof is found that this man has died of poisoning.