Hector Carpentier, the narrator of Louis Bayard's delicious new historical mystery, The Black Tower, is one of those timid souls 'thin and pink and inclined to catch cold''  who is just waiting for a grand adventure to rock his world.

The year is 1818, Hector is 26, and he lives with his maman while nursing a broken heart. You can set your watch by his humdrum routine  until his name and address are discovered on the corpse of a stranger stabbed to death and stripped of his fingernails on a Paris street.

His life is changed by Detective Eugène Vidocq, a real-life crook-turned-cop whose flamboyant history Bayard has embellished with a few juicy new fictional chapters. Depicted here as a shape-shifting, swashbuckling lothario, Vidocq persuades our sad-sack hero to help him solve a mystery: Did Marie Antoinette's son Louis XVII (a.k.a. the Dauphin) really perish in prison in 1795? Or has he survived incognito as a mentally addled country gardener? Bayard's serpentine plot incorporates the long-buried secrets of Hector's late father, a Trojan hobbyhorse, swordplay, and more costume changes than a season of Sex and the City.

 But while Bayard handles this far-fetched material with a light touch, he also manages to imbue his characters with real soul. You may find yourself, more than two centuries after the fact, aching over the fate of the pitiful young Dauphin.