The story deals with the murder of one David Hume, a mysterious individual, who has stolen the plans of a new submarine belonging to the father of Allen Morris. Edith Vale, sweetheart of Morris, notes that her fiancé is the victim of some obsession and through Pendleton, his close friend, enlists the aid of the great detective. Ashton-Kirk takes up the case with a vim, matching his wits against the craft and cunning of the criminal being his hobby. Through a simple little piece of paper made by a conductor's punch, the detective is able to get on the proper scent. This enables him to find his man, who proves to be a mute, and through a note dropped by the latter while in conversation in writing with a friend he learns the perpetrator of the crime. Then, back in the home of Hume, he is secreted when the mute and Sagon enter to recover the plans which Hume had in his possession, and for which the murder was committed.