Nicolas Le Floch is involved in a stormy love affair with Julie de Lastérieux when she's found poisoned in her bed and he becomes the number-one suspect. As he fights to clear his name, the young police commissioner discovers that Mme de Lastérieux was working for the king's secret police. To get Nicolas away from the investigation, Louis XV sends him on a secret mission to London.

Paris is celebrating the Dauphin's coming of age by throwing a fireworks-filled party. Euphoric crowds eagerly await the celebrations, but police commissioner Nicolas Le Floch is worried: the incompetent Major Langlumé is handling security, instead of Royal Police Lieutenant Mr. Sartine. Nicolas' worst fears are confirmed when panicked revelers are crushed to death.