Mademoiselle Yvonne Cartier learns from her brother Bob that he is being cheated out of ownership of a mine by a financier named Jacob Marston She bribes Marstons secretary to take sick leave and is employed in her stead Then with help from her chauffeur Alec she breaks into Marstons safe and steals from it currency bonds share certificates various papers and a small tartan box However just as the job is complete there comes from a neighbouring office the crack of a gun a scream and the thud of a falling body The following morning Marston asks Sexton Blake to investigate the theft