The receipt of a letter from the Italian ambassador finds Holmes, Watson and Mycroft together for a crucial meeting at the embassy. An important member of the Italian Royal family, known for flirting with danger in the anarchists clubs of London is in town. Holmes and Watson are retained to try to keep this Royal alive and a bomb plot tests Holmes ingenuity during a royal visit to Greenwich Observatory.

The stakes are raised when the ambassador invites Holmes and Watson to a Venetian Masked Ball to be held at the embassy. Dressed in their costumes and masks, they have to try and locate two anarchists who have infiltrated the Ball. Holmes has to devise an intiguing way to identify them and Watson has to put his trusty service revolver to good use.