On June 22, 1791, the royal family, which had fled Paris, was spotted in Varennes. In charge of escorting Louis XVI and Marie-Antoinette in their flight, Nicolas Le Floch is a helpless witness to their capture. But his mission isn’t over yet. The queen has unwisely left a terrible secret behind: a coded letter whose contents, if they become known, could precipitate the downfall of the monarchy. She tasks Nicolas with getting it back from the midst of revolutionary Paris, where he will be surrounded by the countless dangers of an explosive political situation.

Nicolas Le Floch’s saga continues through the upheaval of the French Revolution, sorely testing both his bravery and his loyalty to the monarchy, while his friend Bourdeau sides with the new ideas. At the Tuileries, in revolutionary clubs, and at the Palais Royal, Nicolas has to contend with plots hatched by enemies of the Crown. He meets up with Laure de Fitz James, a former lover with shady connections, braves the danger of attempting to infiltrate a royalist secret society with ties to the English throne, and meets a revolutionary young woman who is also a skilled navigator, and who gives him a hand in that risky business…