With one exception, these watchers were never interfered with. Our only mishap, early on, was the arrest of a gingerhaired giant with an entirely Germanic face. This was done by my intervention, largely to placate our nervous landlady, Mrs Hudson. Holmes was not in the house at the time. The man proved to be a plain-clothes sergeant of the Criminal Investigation Division, stationed at Paddington Green police station specifically for our protection. Needless to say, this well-intentioned error on my part delighted my friend.

His letter denouncing the futility of war, from Holmes to the Morning Post, had duly appeared. It was written with sufficient feeling to carry conviction to the unprejudiced reader. I feared we should receive a deluge of hostile post from wartime patriots but such was not the case. We were at an early stage of the conflict and tempers were not as hot as they subsequently became.

Every morning Holmes went by cab to the St James’s Library, off Pall Mall, to resume his work on the counterpoint of Orlando Lassus. The St James’s was a private library, founded by the great John Stuart Mill in the 1840s and restricted to members only. Membership was granted after personal recommendation and election, so that it was easy to check the names of those who made use of it. Holmes the musicologist appeared to work there until late in the afternoon, when he was driven back to Baker Street.

We were informed by Naval Intelligence that those who spied upon the library during the day probably did so from the rooms of an otherwise respectable European Club on the far side of the square. Its windows overlooked the library building. Yet even if these hidden onlookers in St James’s continued to keep watch, they no evidence of Holmes among the readers leaving or returning between his arrival in the morning and his departure for home at the end of the afternoon. They must content themselves with a young carpenter, who had been at work erecting shelves in the reading room, swinging his long bag and whistling as he went on his way to another job. Perhaps a bluff middle-aged fellow in a country suit arrived from Sussex or Surrey for an hour’s browsing among the shelves. An elderly scholar with pince-nez and an old-fashioned stove-pipe hat might appear from the Oxford train or the ecclesiastical figure of an avuncular rural dean would set out for his return to a West Country parish.

Those who have followed the adventures of Sherlock Holmes may at once guess the identity of the whistling carpenter, the bluff countryman, the shrivelled old scholar or the rural dean. Happily it was a fact unknown to German intelligence that in 1879, as an anonymous understudy and at short notice, Holmes had played Horatio to Sir Henry Irving’s Hamlet at the Lyceum. It was his one and only appearance on the London stage. Almost at once he was engaged to sail on an eight-month tour of the United States with the Sasanoff Shakespearean Company. It was not merely that he adopted the costume of the role he played. He assumed the expression, the manner, and the very soul of that part.

Yet I believe my friend considered that he reached the height of impersonation when the cab stopped in the Piccadilly square, the familiar figure strode up the library steps, the driver whipped up the horse and clattered round the corner into Pall Mall. As so often, the spies saw what they expected to see. Yet it was a trustworthy amateur, perhaps resting from the stage, who went up the steps to the library door, while the consulting detective covered by a shabby coat and hat drove the hansom smartly round the corner. Fifteen minutes later, he brought the horse and cab to rest in the securely guarded precincts of Old Admiralty Yard.

If anything more was needed to lay German suspicions to rest, Holmes published in the following spring two impeccable reviews of the polyphony of Orlando Lassus. His learned references to texts and manuscripts were in themselves several pages long. Orders for the journal were placed with Lindemann in neutral Geneva. The destination of two copies proved to be the Bureau of Military Intelligence in Berlin. It was from this moment that a noticeable falling off began in the numbers of those who tracked the hero of Baker Street on his daily journeys to Piccadilly and, presumably, among those who watched forlornly from the windows of the European Club.

4

If my friend expressed his reservations in the first months of the war, when the hopeful belief was that it would be over by Christmas, you may imagine his feelings as the Western Front settled into mud and slaughter. On a pleasant June evening, after almost two years of the conflict, we were sitting either side of our window, discussing the losses of British battle-cruisers in the engagement off Jutland, a fortnight earlier, and the loss of Lord Kitchener, Minister of War, in the sinking of the cruiser HMS Devonshire the previous week. Holmes seemed to be at his lowest ebb.

“I fear that we and Germany may end as two corpses, manacled together,” he said gloomily. After a pause, he stared down into the quiet street and added, “Even were we to defeat the powers of central Europe utterly, the result could only be to destabilise that area completely for fifty years to come.”

“That is something beyond the power of Room 40 to remedy,” I said philosophically.

He stood up and went to the cigarette-box on the mantelpiece. It was a few days before midsummer and the setting sun glowed like molten gold on the far wall of our sitting-room.

He lit his cigarette, shook out the match, and said,

“I cannot make Hall understand that the only way to control German intelligence is to let them read our ciphers.”

As he returned to the window, I wondered if I had heard him correctly. Possibly I had missed a tone of irony in his words. He stood in the golden light, tall and gaunt, emaciated by months of constant work. I was struck by the sudden impatience of his grimace, a growing sense of his consuming energy, an onset of that passionate reasoning power, which I had learnt to recognise with some disquiet.

“You want the Germans to penetrate our codes and ciphers?”

“Of course!” he said emphatically, “We cannot control their thoughts simply by reading their cables. The time has come to let Germany win a battle of the ciphers. It will not do for Hall’s handful of spies to feed them stories of our intentions. That trick is done with. Tirpitz is not a Teutonic clown but the equal of Hall or Fisher. He has been stung too often and will now believe only what he reads for himself.”

“Then what is the answer? Surely not to give away our secrets?”

He shook his head impatiently.

“We must give him the means of reading our codes and ciphers. We must make him feel that he is winning, rather than losing. There has been too much triumph on our side and the braying that goes with it. He must read our wireless messages and signals with the assistance of our own code-books. He must read our confessions of being baffled by his new ciphers.”

“He will not believe any of that!”

To my surprise, he smiled.

“Suppose that we should present him with our Secret Emergency War Code, containing a complete set of our cipher tables for the next six months.”

“He will not believe anything that we give to him!”

“I think he will, if we allow him to steal the emergency code book. You and I could arrange that on our own. I hardly think we need trouble Admiral Hall. Let this be our own enterprise. I believe we could be successful in passing such information to Berlin.”

In that moment my heart seemed to stop.

“I believe we could be shot by our own side!” I said desperately, “Or assassinated by the Germans!”

“My dear Watson, they will be only too happy to believe in the value of a code-book, provided that it is served up to them in the right way. In every neutral country their spies are now ready to pay for whatever information our so-called double-agents betray to them. This is far better. All we need in this case is an apparently indiscreet leakage of our naval and military intelligence, including codes and cipher-tables. I grant you, we shall also need an impersonating agent of our own who must appear simple enough and gullible enough to carry conviction.”