This article details the results of a very long investigation into the life of a character who incarnates the darkest years of French history. Pierre Laval, first a cabinet member and then Council President, was the leader of a collaboration government under German occupation. The research was undertaken in the archives that his son-in-law, Count René de Chambrun, had assembled in his offices and apartment in Paris. It led to the discovery of a new source: the private notebooks that Josée, Pierre Laval's only child, had kept between 1936 and 1992. Once deciphered and analyzed, this source constitutes an extraordinary narrative of the period. It reveals the complicity of a worldly, fashionable milieu that never opened its eyes to the seriousness of what was happening. It reconstitutes the choices and cultural codes of French high society, which submitted meekly to the Nazis. This text emphasizes issues of methodology and the difficulties that writing this story entailed.