Sherlock Holmes and the Secret Weapon (1943) is the fourth in the Basil Rathbone/Nigel Bruce series of Sherlock Holmes films.
Sherlock Holmes (Basil Rathbone) successfully removes Dr. Franz Tobel (William Post Jr.) and his new invention - the "Tobel Bombsight" (analogous to the real-life Norden Bombsight) - from Switzerland to safety in England under the noses of German agents. However once in England, Tobel secretly visits his girlfriend Charlotte Eberli (Kaaren Verne), and is nearly kidnapped outside her home by the henchmen of Holmes' arch-nemesis, Professor Moriarty (Lionel Atwill), who is still alive after being presumed dead and is now in league with the Nazis.
The next day, Tobel successfully demonstrates his new bombsight for Sir Reginald Bailey (Holmes Herbert) and the British air ministry. Back at Whitehall, Tobel tells Sir Reginald that, while he is willing to allow the British to use his invention, only he will know its secret. Tobel then secretly splits his invention into four parts and gives them respectively to four Swiss scientists now living in London. Soon after, Holmes receives a call from Scotland Yard's Inspector Lestrade (Dennis Hoey) telling him that Tobel has disappeared. However Tobel has left a cryptic message in code behind (taken from the Arthur Conan Doyle story The Adventure of the Dancing Men).
Holmes cracks the code and tracks Tobel down, in the process utilizing his skill in disguises, appearing as a Swiss inventor, a criminal Lascar, and an elderly German bookseller.
In the climax of the film Holmes is captured by Moriarty and given his choice of deaths. Holmes opines that it would be curious to have the blood drawn from his body and slowly fade away. Moriarty has a fully equipped operating theatre, so Holmes's idea is soon implemented. A large IV needle, a long rubber tube, and a five-gallon bottle are set up to siphon Holmes's blood out of his body. Fortunately for Holmes, it takes over an hour to die this way, which gives his friends time to find and rescue him: Dr. John H. Watson (Nigel Bruce) raises the blood bottle above Holmes and reverses the siphon flow. Color returns to Holmes's face and he gradually becomes more alert, though he has never lost consciousness. Moriarty tries to escape, but falls to his death because of a trap door deliberately left open by Holmes.
This is the second Basil Rathbone "Sherlock Holmes" film in which Moriarty appears, and also the second one in which he dies (played by George Zucco, Moriarty was thrown to his death from the top of the Tower of London by Holmes in 1939's The Adventures of Sherlock Holmes). In what would probably be considered a ridiculous suspension of disbelief, Moriarty, as played by Henry Daniell, was to return and die yet again (by once more falling accidentally, this time from a drainpipe he has been clinging to on a high building), in The Woman in Green. That latter death of Moriarty was seemingly permanent for this movie series as his death is referenced in the context of having been verified in subsequent titles of the series (e.g., During a scene on the train in Terror By Night, when Holmes says to Inspector Lestrade (Dennis Hoey), "...Colonel Sebastian Moran was the most sinister, ruthless, and diabolically clever henchman of our late but unlamented friend, Professor Moriarty.").
Basil Rathbone – Sherlock Holmes
Nigel Bruce – Dr. Watson
Lionel Atwill – Professor Moriarty
Kaaren Verne – Charlotte Eberli
William Post Jr. - Dr. Franz Tobel
Dennis Hoey – Inspector Lestrade
Holmes Herbert – Sir Reginald Bailey
Mary Gordon – Mrs. Hudson