Sherlock Holmes and the Secret Weapon is the second of the series that's set during World War II, and it features a few hefty dollops of propaganda for its contemporary wartime audience. A Swiss inventor has designed a more accurate bomb sight that needs to be safely delivered into the Allies' hands before Nazi agents can get hold of it and turn the tide of the war. Professor Moriarty features as the prime villain for the second time in the series, and this time round Holmes' arch nemesis is committing treason, tut-tut! What a rotter!

Synopsis

The film opens in Dr. Tobel's home village in Switzerland, and Sherlock Holmes is inside the local café disguised as an elderly German bookseller. He's talking to Nazi agents about their plans to steal the Tobel Bombsight, he's infiltrated their ranks and is helping them; Holmes is posing as a Nazi Agent who wears a bookseller disguise in order to gain access to Dr. Tobel's home. Holmes leaves the café, ostensibly to go and steal the bombsight from Dr. Tobel.

Holmes is let in by Tobel and explains to him that the large books he's carrying have been hollowed out, to accommodate the new bomb-sight which Tobel has split into four parts. Holmes and Tobel then switch places with two of Tobel's servants, the stand-in Holmes and Tobel then leave the house. The Nazis fall for the ruse and they follow the decoys, while Holmes gets Dr. Tobel out of the house and across the border in the agents' own car.

Back in 221B Baker St., Holmes leaves Tobel under Dr. Watson's watchful eye while he goes to see Sir Reginald Bailey. Watson falls asleep in front of the fire, and Tobel slips out to go and see his girlfriend, Charlotte Eberli. At her flat, he writes Holmes a coded message that's to be opened if anything should happen to him. On his return to Baker St., Tobel is attacked in the street but luckily, he's saved by a police constable.

Next morning, the bombsight is tested at Salisbury Plain with great success. Holmes, Watson and Tobel go to see Sir Reginald where Tobel announces that he will not be handing over the bombsight for reproduction, rather he'll be building them himself with the help of four scientists known only to him. Sir Reginald is non-to-pleased but has little choice to let Tobel carry through his plan.

Tobel manages to deliver the designs for the four parts to four individual scientists before he disappears. Holmes and Watson search Tobel's workshop and detect the scent of perfume, which leaves them to Charlotte Eberli, who tells Holmes of the message Tobel left behind for him. Unfortunately, someone has already taken the message and left a taunting note of their own in its place. From Charlotte's description of a workman that had paid her a visit, Holmes realises that he is up against his arch nemesis, Professor Moriarty.

Disguised as a murderous lascar of Moriarty's previous employ, Holmes searches the drinking dens of Soho and with the help of a local denizen called Peg Leg, Holmes tracks down Moriarty. The Professor is lying in wait for an unsurprised Holmes at Jack Brady's Carpenter's Shop, he's after the four pieces of the bomb sight so he can sell it to the Nazis for a very large sum indeed. After some verbal fencing, Moriarty binds and stuffs Holmes into a sea chest equipped with a false bottom, to be disposed of at sea by Jack Brady. Brady and Peg Leg are carrying the chest when Watson and Lestrade question them and soon realise that something is amiss. Brady and Peg Leg peg it and leave a very uncomfortable Holmes to be retrieved safely.

Holmes goes back to Charlotte Eberli's to question here again and discovers that the message that Tobel wrote for him was written with a pencil on a notepad, and luckily, the indentations are still on the top sheet. Holmes returns to Baker St. and there, he chemically treats the sheet to make the indentations legible. The message is encrypted using a series of 'dancing men'; stick figures in many different positions. Holmes soon cracks the cipher, save the last line of the message. Holmes now has the name and whereabouts of three of the four scientists that Dr. Tobel enlisted to help him.

Holmes and Watson arrive at the home of the first scientist on the list, Jacob Durrer, to find that Lestrade is already on the scene. Durrer has been murdered and when Holmes prompts Lestrade to telephone Scotland Yard, the Inspector is told that two more of the other scientists on the list are also dead. The race is on with Moriarty to decipher the last line of Tobel's message. Moriarty's men torture Tobel but to no avail, he won't reveal the name of the last scientist. At Baker St., Watson again inadvertently saves the day as he complains about the coded message's final line and Holmes is inspired to decipher it in a flash. The fourth scientist is Prof. Frederick Hoffner of Sloane Square.

By way of a spilt glass of water, Moriarty also cracks the final line of the cipher and so his men pay a visit to the Prof. Hoffner. Moriarty abducts Hoffner but when he get him back to his headquarters, the scientist reveals himself to be Holmes in disguise. Meanwhile, Watson is following Holmes' instructions and is following the trail of Moriarty's gang; the real Hoffner has attached a small apparatus that drips a trail of luminous paint on the road, which will lead them to Moriarty's door.

Moriarty offers Holmes his choice of death, so Holmes asks to have the blood drawn from his body and slowly fade away. Coincidentally, Moriarty has a fully equipped operating theatre, so the request becomes reality and an intravenous needle siphon is set up to draw out Holmes' blood. When Watson and Lestrade arrive at the hideout, the police slip inside and rescue Dr. Tobel while Moriarty is busy attending to Holmes. Watson and Lestrade then go in alone.

Fortunately, it takes over an hour to die in the way Holmes chose and so upon discovering the emaciated detective, Watson raises the blood drip above Holmes' arm and reverses the flow. Colour returns to Holmes' face and he wakes up. Moriarty escapes temporarily through a secret passage behind a secure panel but he falls to his death through his own trap door, which was deliberately left open by Holmes.

The film closes with all the main cast watching British planes flying overhead, on their way to Germany, with Tobel and the military brass saying some patriotic words and Holmes quoting Shakespeare's Richard II.