Neville St Clair works in the City of London and is a man of very regular and respectable habits. One day, his wife goes into town for lunch and is shocked to see her husband in the window of an opium den. That evening, he fails to return home, and Mrs St Clair decides to consult Holmes.

The body of one Cadogan West is found in an underground tunnel near Aldgate Station, with his skull smashed, and Holmes is puzzled by the interest his brother Mycroft is showing in the case. Mycroft explains that the top secret plans for a new submarine were in the dead man's pocket, but three of the ten pages were missing when the body was found. Mycroft, on behalf of the British Government, asks his brother to find the missing pages.

The Mazarin Stone, a famous jewel, acts as a magnet for crime. Holmes investigates the latest and perhaps most curious episode in its adventures.