France, 1860: In Bougival, a small town near Paris, the widow Lerouge is found stabbed to death. No one knows who the murderer is yet, but investigations reveal that 30 years ago, Madame Lerouge, as a wet nurse, swapped the illegitimate son of Count Commarin for the legitimate heir. This means that Noel Gerdy, a lawyer who grew up in poverty, is the count's legitimate son—and now the prime suspect in the murder.