A man has been killed in the center of Paris. A dirty piece of paper with the address of Dr. Hector Carpentier was found in his pocket. Having begun to investigate the murder, the famous French detective Eugene Francois Vidocq turns to the only doctor in Paris with that name. It soon becomes clear that the crime is connected with one of the most mysterious mysteries in French history - the fate of the young Dauphin Louis Charles, the son of Marie Antoinette and King Louis XVI. According to the official version, the Dauphin suffered a terrible death in the terrifying Parisian dungeon known as the Black Tower. But what if the boy survived?