At the end of the nineteenth century, serial murderer Joseph Vacher, dubbed "The Killer of Little Shepherds," terrorized the French countryside. He eluded authorities for years-until he ran up against prosecutor Emile Fourquet and Dr. Alexandre Lacassagne, the era´s most renowned criminologist. The two men typified the Belle Epoque, a period of immense scientific achievement and fascination with its promise to reveal the secrets of the human condition.