Like many fin de siècle Parisians, Inspector Achille Lefebvre is looking forward to a pleasant summer holiday at a seaside resort with his wife, Adele—but a body found hanging from a bridge in a public park interferes with the inspector's plans.

Paris: July, 1890. Inspector Achille Lefebvre and his wife Adele are enjoying their stay at a seaside resort—until a body found hanging from a bridge in a public park demands the Inspector's attention.

Is it suicide or murder? A twisted trail of evidence draws Inspector Lefebvre into a shadowy underworld of international intrigue, espionage, and terrorism. Time is of the essence; pressure mounts on the Sûreté to get results. Achille's chief orders him to work with his former partner, Inspector Rousseau, now in charge of a special unit in the newly formed political brigade. But can Achille trust the detective who let him down in another case?

Inspector Lefebvre uses innovative forensics and a network of police spies to uncover a secret alliance, a scheme involving the sale of a cutting-edge high explosive, and an assassination plot that threatens to ignite a world war.