“Admirable!” cried Holmes in appreciation.

“I took the pile of papers from the safe,” continued Armstrong, “and as I did so my mouth was dry. I knew I could not be mistaken. As I lifted up the sheets one by one, my fingers were trembling: page one, page two, page three, page four, page five. The pages were in perfect order. At once a lump came into my throat, and I thought I would faint.”

“Because you had intentionally left the pages in a different order,” remarked Holmes.

“Precisely,” said Armstrong, his face aghast as he relived the episode. “It is an eccentric habit of mine always to place page five above page four in such a pile. I do it quite deliberately. Clearly someone had taken the pile of papers from the safe, examined them and probably copied them. He had then replaced them in the correct order – the order in which he had examined them – not realizing that I had deliberately left them in an incorrect order.”

“Could you possibly be mistaken on the point?” Holmes interrupted, but Armstrong shook his head.

“There is no doubt in my mind,” said he. “Besides, I also make a habit of turning down the top right-hand corner of page seven, and that, I saw, had been smoothed flat. I realized with a sick feeling in my stomach why it appeared to have taken Norton so long to copy out the Baltic papers. No doubt he had spent most of my two hours’ absence copying out the Army’s secret codes and ciphers. I can only assume that he intends to pass them to the enemies of this country, who will thus be privy to our most secret commands and communications. The country will be laid open to attack, and I shall certainly be court-martialled!”

“What action have you taken?” asked Holmes.

“None.”

“None?”

“I am confused as to what I should do. The whole affair is utterly impossible! It is impossible for anyone but Norton to have opened the safe, for no one but he was in the office while I was out. But it is equally impossible for Norton himself to have done so, for he does not have a key!”

“Who does have a key?”

“There are just four, and each man who holds one pledges to defend it with his life. I have one, which never leaves my possession. It is on my watch-chain now, as you see. Major Lavelle, of course, has one, and one is held by each of the other two senior officers seconded to the department at the moment, Colonel Fitzwarren and Admiral Pettigrew.

“When I realized what had happened and saw the difficulty in the matter, I could not think where to seek advice. Major Lavelle does not return from Portsmouth until tomorrow afternoon. Eventually, I went round to Colonel Fitzwarren’s club in Pall Mall, sure that I would find him there at that time, but I was informed that he had left some time previously, and I have been unable to find him. Admiral Pettigrew, I was aware, had an appointment with the Chancellor of the Exchequer earlier this evening, and I thought it better not to interrupt them. It was then that I thought of you, Mr Holmes. You must advise me, as you advised Major Colefax. I put the case entirely in your hands, and beg you to help, not for the sake of my miserable career, but for the sake of the country!”

“When you have eliminated the impossible,” said Holmes after a moment, “whatever remains, however improbable, must be the truth. If your memory of how you left the papers in the safe is correct, which I do not doubt, then, as no one else had been in the room, Norton must be guilty of treachery, either alone or in company with another.”

“But the matter of the key— ”

“Is a lesser problem. Norton may have secured a copy— ”

“Impossible!”

“Or been lent a key by one of your colleagues.”

“Inconceivable!”

“Unlikely, perhaps, but not impossible, Captain Armstrong! Where does Norton live?”

“Trevor Place, on the south side of Hyde Park.”

“Then we must go there at once!”

“I have no real evidence against him.”

“Never fear, Captain Armstrong. If Norton is guilty, we shall find evidence!”

In a minute we were in Armstrong’s carriage, in which two marines were waiting, and were making our way slowly through the snowbound streets. The night was a cold one, and the snow was falling heavily now, the tumbling snowflakes almost obliterating the feeble glimmer of the street lamps.

It was no great distance to Norton’s house, but our progress was slow, as the horse slithered and slipped upon the snow. The streets were almost deserted, and we passed only a single vehicle along the entire length of Park Lane and Knights-bridge. As we turned into Trevor Place, however, we passed a cab going in the opposite direction, its progress as slow as our own.

We pulled up before one of the small flat-fronted houses, towards the bottom of the street. Holmes sprang out quickly and examined the ground, a frown on his face.

“That cab we passed came from this house,” said he in a thoughtful tone, pointing to the churned-up snow by the kerb. “It would be worth something to know who was in it!”

Our knock at the door was answered by a manservant, who showed us into a small drawing room. A thin, dark-haired man wearing a pair of gold-rimmed spectacles was sitting by the fire, reading a newspaper, but he sprang up as we entered. There was, I thought, a look of alarm upon his features.

“Captain Armstrong!” said he in a breathless tone. “What a pleasant surprise!”

“Not so pleasant,” returned the other. “I must warn you, Norton, that you are under suspicion of interfering with confidential documents.”

“What nonsense!” cried Norton, casting his newspaper down and standing defiantly with his hands upon his hips. He listened as Armstrong outlined his suspicions, a forced smile fixed upon his face all the time. “I see you have no evidence whatever against me,” he retorted as Armstrong finished.

“Never mind that,” interrupted Holmes. “You have had a visitor here this evening.”

Norton shook his head. “On the contrary,” he replied, “I have been quite alone, until you arrived.”

“Then how do you explain the presence of these two glasses upon the side table?” said Holmes. He picked them up as he spoke and sniffed each in turn. “This first one has had brandy in it, and this one a whisky mixture. You cannot pretend they are both yours.”

Norton hesitated a moment before replying.

“Oh, very well,” said he at length, in a tone of annoyance. “An old friend of mine called by.”

“His name?”

“That is none of your business and I refuse to say.”

“Then we shall ask your servant.”

“Flegge? By all means,” answered Norton in a careless tone, giving the bell-rope a tug. “Ask him what you please.”

The manservant entered and the situation was explained to him, but he could tell us very little, for he had not seen the visitor. Norton had opened the front door himself, and when Flegge had brought in the drinks, had taken the tray from his hand at the door.

“Perhaps you heard the gentlemen speaking?” Holmes suggested.

“A very few words, sir. They stopped talking as I opened the door.”

“What were the words you heard?”

“A mention of Princess Zelda.”

“Nine-thirty-five,” came an odd voice, hoarse and croaking, from behind us.

I looked round sharply. In the corner of the room, on a wooden perch, stood a small grey parrot. My attention had been so focused upon Norton since we had entered the room that I had not noticed it before. I glanced back at Norton. There was a flicker of fear in his eye. Holmes evidently saw it, too, for after a moment’s thought he approached the parrot.

“What did you say?” said he.

The bird tilted its head on one side and regarded him with a disconcertingly intelligent eye, but remained silent.

“Princess Zelda,” Holmes tried again.

“Nine-thirty-five,” responded the parrot promptly in a clear tone.

“Nine-thirty-five?” Holmes repeated.