Sir Arthur Conan Doyle’s “The Adventure of the Dancing Men” inspired The Secret Weapon. The wartime plot pits Sherlock Holmes once more against his nemesis, the villainously brilliant Professor Moriarty, who was believed dead but is now working for the Nazis. The Nazis have assigned Moriarty to kidnap Dr. Franz Tobel, the inventor of a new super bombsight. Sherlock Holmes outwits the enemy agents and escapes with Tobel and his precious invention. But despite elaborate precautions, the inventor later disappears before the process of manufacturing the bombsight is perfected. Holmes and Watson must stop the Nazis from getting their hands on the new bombsight, wrapped in a code of dancing men. Using a variety of disguises-a Swiss inventor, the Lascar sailor Ram Singh, and an old German bookseller, Holmes puts his own life on the line in a race against the clock to prevent Moriarty from carrying out his evil plans.

The Secret Weapon is the first of the films to introduce Dennis Hoey as Scotland Yard detective Inspector Lestrade.