Louis Mennechain is a modest locksmith. So imagine his amazement when Professor Piétrefond approaches him and tells him he recognizes in him a descendant of the kings of France, more precisely of Louis XVII (the king who never reigned). According to him he is none other than the monarch's great grandson. Louis's life is soon turned upside down as he becomes the center of attention of a group of ardent royalists. Even a duchess "gives" him her love to make sure that he accepts his role as pretender to the throne...

Grâce à l'intervention d'un historien-archiviste nommé Piétrefond, l'honnête serrurier Louis Mennechain est fêté comme le descendant de Louis XVI par une brillante duchesse et son entourage. Ébloui, par sa nouvelle condition, Mennechain abandonnerait sa fiancée si Pietrefond ne lui révélait son escroquerie.