“Its importance can hardly be exaggerated. It has been the most jealously guarded of all government secrets. You may take it from me that naval warfare becomes impossible within the radius of a Bruce-Partington’s operation. Two years ago a very large sum was smuggled through the Estimates and was expended in acquiring a monopoly of the invention. Every effort has been made to keep the secret. The plans, which are exceedingly intricate, comprising some thirty separate patents, each essential to the working of the whole, are kept in an elaborate safe in a confidential office adjoining the arsenal, with burglar-proof doors and windows. Under no conceivable circumstances were the plans to be taken from the office. If the chief constructor of the Navy desired to consult them, even he was forced to go to the Woolwich office for the purpose. And yet here we find them in the pocket of a dead junior clerk in the heart of London. From an official point of view it’s simply awful.”

“But you have recovered them?”

“No, Sherlock, no! That’s the pinch. We have not. Ten papers were taken from Woolwich. There were seven in the pocket of Cadogan West. The three most essential are gone - stolen, vanished. You must drop everything, Sherlock. Never mind your usual petty puzzles of the police-court. It’s a vital international problem that you have to solve. Why did Cadogan West take the papers, where are the missing ones, how did he die, how came his body where it was found, how can the evil be set right? Find an answer to all these questions, and you will have done good service for your country.”

“Why do you not solve it yourself, Mycroft? You can see as far as I.”

“Possibly, Sherlock. But it is a question of getting details. Give me your details, and from an armchair I will return you an excellent expert opinion. But to run here and run there, to cross-question railway guards, and lie on my face with a lens to my eye - it is not my métier. No, you are the one man who can clear the matter up. If you have a fancy to see your name in the next honours list- -”

My friend smiled and shook his head.

“I play the game for the game’s own sake,” said he. “But the problem certainly presents some points of interest, and I shall be very pleased to look into it. Some more facts, please.”

“I have jotted down the more essential ones upon this sheet of paper, together with a few addresses which you will find of service. The actual official guardian of the papers is the famous government expert, Sir James Walter, whose decorations and sub-titles fill two lines of a book of reference. He has grown gray in the service, is a gentleman, a favoured guest in the most exalted houses, and, above all, a man whose patriotism is beyond suspicion. He is one of two who have a key of the safe. I may add that the papers were undoubtedly in the office during working hours on Monday, and that Sir James left for London about three o’clock taking his key with him. He was at the house of Admiral Sinclair at Barclay Square during the whole of the evening when this incident occurred.”

“Has the fact been verified?”

“Yes; his brother, Colonel Valentine Walter, has testified to his departure from Woolwich, and Admiral Sinclair to his arrival in London; so Sir James is no longer a direct factor in the problem.”

“Who was the other man with a key?”

“The senior clerk and draughtsman, Mr. Sidney Johnson. He is a man of forty, married, with five children. He is a silent, morose man, but he has, on the whole, an excellent record in the public service. He is unpopular with his colleagues, but a hard worker. According to his own account, corroborated only by the word of his wife, he was at home the whole of Monday evening after office hours, and his key has never left the watch-chain upon which it hangs.”

“Tell us about Cadogan West.”