A man named Adolphe visits Leroux at his office at the Morning Journal and delivers to him a chest that was bequeathed to him by his friend Theophraste Longuet. Leroux begins investigating its contents and relates the very strange experience of Longuet.
While visiting a famous prison, Theophraste Longuet, a retired rubber stamp manufacturer, is possessed by the spirit of Cartouche, a criminal who plagued Paris in the eighteenth century. This wicked spirit overcomes Longuet more and more frequently, and Longuet begins, unwittingly, to commit the crimes for which Cartouche was known in the past.