Prudence and Bélisaire Beresford have decided to take a well-deserved vacation. But a wealthy Russian heiress disappears and Prudence can't resist the call of danger. Bélisaire is forced to follow his rebellious wife. The investigation will lead them on the trail of a mysterious scientest who possesses the secret of eternal youth. The third part of the adventures of the famous Beresford couple created by Agatha Christie, this new adventure directed by Pascal Thomas, and starring Catherine Frot and André Dussollier, combines all the ingredients of a detective film and a detective "fantasy," with startling forays into screwball comedy.