Sherlock Holmes and the Secret Weapon is a 1943 film, the fourth film in the Sherlock Holmes film series starring Basil Rathbone as Sherlock Holmes and Nigel Bruce as Dr John Watson. Lionel Attwill stars as Professor Moriarty and Dennis Hoey stars as Inspector Lestrade. The film is essentially British propaganda intended to boost morale in the war against Nazi-German Empire. The film has passed into the public domain.
The film is credited as an adaptation of Sir Arthur Conan Doyle's The Adventure of the Dancing Men, though there is little resemblance beyond the inclusion of the Dancing Men code. Set during World War II, the film tells of a case in which Holmes and Watson thwart the theft of an advanced bomb-sight that could win Britain the war.
In Switzerland, Nazi spies plan to abduct Dr Franz Tobel, inventor of a new bombsight, and bring him to Berlin. Sherlock Holmes, disguised as a German bookseller, manages to escape Gestapo arrest with Tobel who intends to offer his invention to the British war effort. Holmes keeps Tobel in Baker Street under the protection of his companion Dr Watson. Against Holmes' wishes, Tobel sneaks out to meet with his lover Charlotte Eberli and gives her a coded message which she is to give to Holmes if something should happen to him. On his way back, he is attacked in a failed kidnap attempt. He returns to Baker Street with minimal injuries.
Tobel successfully demonstrates the capabilities of his bombsight to the British government, Holmes, Sir Reginald Bailey and Inspector Lestrade, but he insists that the intricacies of his invention are known only to him; the only condition of his offer. He secretly splits the bombsight into four pieces, each useless without the other three, and sends them each to four scientists unaware of each other's identities. Tobel mysteriously vanishes, and Holmes' deductions lead him to Miss Eberli, who gives him the message Tobel wrote. However, the message has disappeared, replaced with a message from Holmes' own nemesis - Professor James Moriarty, who is believed to have been dead for many years.
Holmes disguises himself as Ram Singh, one of Moriarty's former henchmen, in a scheme to find and confront Moriarty himself. Upon meeting Moriarty, Holmes is close to being killed by Moriarty's men by being hidden in the false bottom of a chest to be thrown into the sea, but is saved by Lestrade's uncharacteristic quick thinking. Holmes returns to Miss Eberli and inspects the impressions of the notepad Tobel had written his message on. Immesring it in fluorescent salts and photographing it in ultraviolet light, the broken fibres of the paper reveal a set of "dancing men" in an alphabet substitution code, which identify the four scientists Tobel gave the pieces of the bomb sight to. Holmes and Watson deduce all but one of the ciphers, which has a different code as an added precaution. Moriarty quickly kills three of the scientists and obtains the parts of the bombsight, his torture techniques failing to break Tobel's secrecy. Holmes and Moriarty simultaneously break the fourth code, identifying the final scientist as Fredrich Hoffner.
The Hoffner Moriarty's men abduct is actually Sherlock Holmes in disguise, revealing that Holmes has safely hidden the real Hoffner. Unknown to Moriarty, the car used to abduct Holmes was fitted with an apparatus that drips paint, leaving a trail to Moriarty's stronghold for Lestrade, Watson and the police to follow. Holmes tricks Moriarty into attempting to bleed him to death in order to stall him before Watson arrives. Watson saves Holmes before he loses too much blood and Lestrade's men defeat Moriarty's henchmen and save Tobel. When Moriarty attempts to flee through a secret passageway, he falls sixty feet to his death into the sewers below, as Holmes discovered the trap door intended to kill himself and left it open. The characters witness another demonstration of the bombsight which is back in the hands of Britain and out of the hands of Hitler's Germany, and Dr Tobel is back to safety. The film ends with a positive and patriotic tone;
"Things are looking up, Holmes," says Watson. "This little island is still on the map."
to which Holmes replies "Yes. This fortress built by nature for herself. This blessed plot. This earth, this realm. This England." (He is paraphrasing from Shakespeare's "King John")