When five members of the cheery, churchgoing Dupont de Ligonnès family were found dead at their home in Nantes – buried in the garden with small Catholic relics – a million questions erupted from neighbours, family and friends.

But for the police, only one question mattered: where was the sixth? Xavier Dupont de Ligonnès was nowhere to be found. And as they dug into his past, they uncovered a story that’s obsessed the nation of France for years: one of an ancient aristocratic dynasty, of international espionage, and a crime scene that, decades later, still refuses to add up…

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