On their first night in Paris, the Gamaches gather as a family for a bistro dinner with Armand's godfather, billionaire Stephen Horowitz. Walking home together after the meal, they watch in horror as Stephen is knocked down and critically injured in what Gamache knows is a deliberate attempt on the elderly man's life. When a strange key is found in Stephen's possession it sends Armand, his wife Reine-Marie, and his former second-in-command at the Surete, Jean-Guy Beauvoir, from the top of the Tour d'Eiffel, to the bowels of the Paris Archives, from luxury hotels to odd, coded, works of art. It sends them deep into the secrets Armand's godfather has kept for decades. A gruesome discovery in Stephen's Paris apartment makes it clear the secrets are deeper and the danger far greater, than they realized. Soon the whole family is caught up in a web of lies and deceit. Gamache will have to decide whether he can trust his friends, his colleagues, his instincts, his own past. His own family. For even the City of Light casts long shadows. And in that darkness devils hide.