The receipt of a mysterious package, sent to Holmes from Pentonville prison, begins a case that should it not be resolved, our great nation with its proud naval tradition, might founder.
A brilliant naval engineer, Benjamin Stewart, who has been detained for his own protection, contacts Holmes in an ingenious manner, asking for his help. However, before Holmes can take up the case, the man is murdered. In his message to Holmes, Stewart has implicated his brother who resides at an unknown address in Chiswick and it is now a concern that the brother's life may also be in danger. Stewart's message contained a final cryptic clue, namely, "The diver has the answer in the devils claw". It is this conundrum that Holmes believes is the key to the development of the steam turbine demonstrated by the vessel 'Turbinia' at the Spithead naval review.
Mycroft reveals that other nations are eager to learn its secrets and Holmes is inflamed when he discovers that the contents of Stewart's message have been stolen from Baker Street. He is greatly relieved when Watson, inadvertently, provides Holmes with the key to the puzzle. After a brief visit to Kew by Holmes, they travel to Chiswick where the diver's secret is finally revealed.