The gruesome murder of a young French physician draws aristocratic investigator Sebastian St. Cyr and his pregnant wife, Hero, into a dangerous, decades-old mystery as a wrenching piece of Sebastian' s past puts him to the ultimate test. Regency England, January 1813: When a badly injured Frenchwoman is found beside the mutilated body of Dr. Damion Pelletan in one of London' s worst slums, Sebastian finds himself caught in a high-stakes tangle of murder and revenge.

Although the woman, Alexi Sauvage, has no memory of the attack, Sebastian knows her all too well from an incident in his past - an act of wartime brutality and betrayal that nearly destroyed him. As the search for the killer leads Sebastian into a treacherous web of duplicity, he discovers that Pelletan was part of a secret delegation sent by Napoleon to investigate the possibility of peace with Britain. Despite his powerful father-in-law' s warnings, Sebastian plunges deep into the mystery of the " Lost Dauphin," the boy prince who disappeared in the darkest days of the French Revolution, and soon finds himself at lethal odds with the Dauphin' s sister - the imperious, ruthless daughter of Marie Antoinette - who is determined to retake the French crown at any cost.

With the murderer striking ever closer, Sebastian must battle new fears about Hero' s health and that of their soon-to-be born child. When he realizes the key to their survival may lie in the hands of an old enemy, he must finally face the truth about his own guilt in a past he has found too terrible to consider.