I collected some baggage and a pile of newly arrived letters from my rooms in Westminster and took a taxi to my Park Lane flat. Usually I had gone back to that old place with a great feeling of comfort, like a boy from school who ranges about his room at home and examines his treasures. I used to like to see my hunting trophies on the wall and to sink into my own armchairs But now I had no pleasure in the thing. I had a bath, and changed into uniform, and that made me feel in better fighting trim. But I suffered from a heavy conviction of abject failure, and had no share in Macgillivray's optimism. The awe with which the Black Stone gang had filled me three years before had revived a thousandfold. Personal humiliation was the least part of my trouble. What worried me was the sense of being up against something inhumanly formidable and wise and strong. I believed I was willing to own defeat and chuck up the game.

Among the unopened letters was one from Peter, a very bulky one which I sat down to read at leisure. It was a curious epistle, far the longest he had ever written me, and its size made me understand his loneliness. He was still at his German prison-camp, but expecting every day to go to Switzerland. He said he could get back to England or South Africa, if he wanted, for they were clear that he could never be a combatant again; but he thought he had better stay in Switzerland, for he would be unhappy in England with all his friends fighting. As usual he made no complaints, and seemed to be very grateful for his small mercies. There was a doctor who was kind to him, and some good fellows among the prisoners.

But Peter's letter was made up chiefly of reflection. He had always been a bit of a philosopher, and now, in his isolation, he had taken to thinkin hard, and poured out the results to me on pages of thin paper in his clumsy handwriting. I could read between the lines that he was having a stiff fight with himself. He was trying to keep his courage going in face of the bitterest trial he could be called on to face - a crippled old age. He had always known a good deal about the Bible, and that and the Pilgrim's Progress were his chief aids in reflection. Both he took quite literally, as if they were newspaper reports of actual recent events.

He mentioned that after much consideration he had reached the conclusion that the three greatest men he had ever heard of or met were Mr Valiant-for-Truth, the Apostle Paul, and a certain Billy Strang who had been with him in Mashonaland in '92. Billy I knew all about; he had been Peter's hero and leader till a lion got him in the Blaauwberg. Peter preferred Valiant-for-Truth to Mr Greatheart, I think, because of his superior truculence, for, being very gentle himself, he loved a bold speaker. After that he dropped into a vein of self-examination. He regretted that he fell far short of any of the three. He thought that he might with luck resemble Mr Standfast, for like him he had not much trouble in keeping wakeful, and was also as 'poor as a howler', and didn't care for women. He only hoped that he could imitate him in making a good end.

Then followed some remarks of Peter's on courage, which came to me in that London room as if spoken by his living voice. I have never known anyone so brave, so brave by instinct, or anyone who hated so much to be told so. It was almost the only thing that could make him angry. All his life he had been facing death, and to take risks seemed to him as natural as to get up in the morning and eat his breakfast. But he had started out to consider the very thing which before he had taken for granted, and here is an extract from his conclusions. I paraphrase him, for he was not grammatical.

It's easy enough to be brave if you're feeling well and have food inside you. And it's not so difficult even if you're short of a meal and seedy, for that makes you inclined to gamble. I mean by being brave playing the game by the right rules without letting it worry you that you may very likely get knocked on the head. It's the wisest way to save your skin. It doesn't do to think about death if you're facing a charging lion or trying to bluff a lot of savages. If you think about it you'll get it; if you don't, the odds are you won't. That kind of courage is only good nerves and experience ... Most courage is experience. Most people are a little scared at new things ...

You want a bigger heart to face danger which you go out to look for, and which doesn't come to you in the ordinary way of business. Still, that's Pretty much the same thing - good nerves and good health, and a natural liking for rows. You see, Dick, in all that game there's a lot Of fun. There's excitement and the fun of using your wits and skill, and you know that the bad bits can't last long. When Arcoll sent me to Makapan's kraal I didn't altogether fancy the job, but at the worst it was three parts sport, and I got so excited that I never thought of the risk till it was over ...

But the big courage is the cold-blooded kind, the kind that never lets go even when you're feeling empty inside, and your blood's thin, and there's no kind of fun or profit to be had, and the trouble's not over in an hour or two but lasts for months and years. One of the men here was speaking about that kind, and he called it 'Fortitude'. I reckon fortitude's the biggest thing a man can have - just to go on enduring when there's no guts or heart left in you. Billy had it when he trekked solitary from Garungoze to the Limpopo with fever and a broken arm just to show the Portugooses that he wouldn't be downed by them. But the head man at the job was the Apostle Paul ...

Peter was writing for his own comfort, for fortitude was all that was left to him now. But his words came pretty straight to me, and I read them again and again, for I needed the lesson. Here was I losing heart just because I had failed in the first round and my pride had taken a knock. I felt honestly ashamed of myself, and that made me a far happier man. There could be no question of dropping the business, whatever its difficulties. I had a queer religious feeling that Ivery and I had our fortunes intertwined, and that no will of mine could keep us apart. I had faced him before the war and won; I had faced him again and lost; the third time or the twentieth time we would reach a final decision. The whole business had hitherto appeared to me a trifle unreal, at any rate my own connection with it. I had been docilely obeying orders, but my real self had been standing aside and watching my doings with a certain aloofness. But that hour in the Tube station had brought me into the serum, and I saw the affair not as Bullivant's or even Blenkiron's, but as my own. Before I had been itching to get back to the Front; now I wanted to get on to Ivery's trail, though it should take me through the nether pit. Peter was right; fortitude was the thing a man must possess if he would save his soul.

The hours passed, and, as I expected, there came no word from Macgillivray. I had some dinner sent up to me at seven o'clock, and about eight I was thinking of looking up Blenkiron. just then came a telephone call asking me to go round to Sir Walter Bullivant's house in Queen Anne's Gate.

Ten minutes later I was ringing the bell, and the door was opened to me by the same impassive butler who had admitted me on that famous night three years before. Nothing had changed in the pleasant green-panelled hall; the alcove was the same as when I had watched from it the departure of the man who now called himself Ivery; the telephone book lay in the very place from which I had snatched it in order to ring up the First Sea Lord. And in the back room, where that night five anxious officials had conferred, I found Sir Walter and Blenkiron.

Both looked worried, the American feverishly so. He walked up and down the hearthrug, sucking an unlit black cigar.

'Say, Dick,' he said, this is a bad business. It wasn't no fault of yours. You did fine. It was us - me and Sir Walter and Mr Macgillivray that were the quitters.'

'Any news?' I asked.

'So far the cover's drawn blank,' Sir Walter replied. 'It was the devil's own work that our friend looked your way today. You're pretty certain he saw that you recognized him?'

'Absolutely. As sure as that he knew I recognized him in your hall three years ago when he was swaggering as Lord Alloa.'

'No,' said Blenkiron dolefully, that little flicker of recognition is just the one thing you can't be wrong about. Land alive! I wish Mr Macgillivray would come.'

The bell rang, and the door opened, but it was not Macgillivray. It was a young girl in a white ball-gown, with a cluster of blue cornflowers at her breast. The sight of her fetched Sir Walter out of his chair so suddenly that he upset his coffee cup.

'Mary, my dear, how did you manage it? I didn't expect you till the late train.'

'I was in London, you see, and they telephoned on your telegram. I'm staying with Aunt Doria, and I cut her theatre party. She thinks I'm at the Shandwick's dance, so I needn't go home till morning ... Good evening, General Hannay. You got over the Hill Difficulty.'

'The next stage is the Valley of Humiliation,' I answered.

'So it would appear,' she said gravely, and sat very quietly on the edge of Sir Walter's chair with her small, cool hand upon his.

I had been picturing her in my recollection as very young and glimmering, a dancing, exquisite child. But now I revised that picture. The crystal freshness of morning was still there, but I saw how deep the waters were. It was the clean fineness and strength of her that entranced me. I didn't even think of her as pretty, any more than a man thinks of the good looks of the friend he worships.

We waited, hardly speaking a word, till Macgillivray came. The first sight of his face told his story.

'Gone?' asked Blenkiron sharply. The man's lethargic calm seemed to have wholly deserted him.

'Gone,' repeated the newcomer. 'We have just tracked him down. Oh, he managed it cleverly. Never a sign of disturbance in any of his lairs. His dinner ordered at Biggleswick and several people invited to stay with him for the weekend - one a member of the Government. Two meetings at which he was to speak arranged for next week. Early this afternoon he flew over to France as a passenger in one of the new planes. He had been mixed up with the Air Board people for months - of course as another man with another face. Miss Lamington discovered that just too late. The bus went out of its course and came down in Normandy. By this time our man's in Paris or beyond it.'

Sir Walter took off his big tortoiseshell spectacles and laid them carefully on the table.

'Roll up the map of Europe,' he said. 'This is our Austerlitz. Mary, my dear, I am feeling very old.'

Macgillivray had the sharpened face of a bitterly disappointed man. Blenkiron had got very red, and I could see that he was blaspheming violently under his breath. Mary's eyes were quiet and solemn. She kept on patting Sir Walter's hand. The sense of some great impending disaster hung heavily on me, and to break the spell I asked for details.

'Tell me just the extent of the damage,' I asked. 'Our neat plan for deceiving the Boche has failed. That is bad. A dangerous spy has got beyond our power. That's worse. Tell me, is there still a worst? What's the limit of mischief he can do?'

Sir Walter had risen and joined Blenkiron on the hearthrug. His brows were furrowed and his mouth hard as if he were suffering Pain.

'There is no limit,' he said. 'None that I can see, except the long- suffering of God. You know the man as Ivery, and you knew him as that other whom you believed to have been shot one summer morning and decently buried. You feared the second - at least if you didn't, I did - most mortally. You realized that we feared Ivery, and you knew enough about him to see his fiendish cleverness. Well, you have the two men combined in one man. Ivery was the best brain Macgillivray and I ever encountered, the most cunning and patient and long-sighted. Combine him with the other, the chameleon who can blend himself with his environment, and has as many personalities as there are types and traits on the earth. What kind of enemy is that to have to fight?'

'I admit it's a steep proposition. But after all how much ill can he do? There are pretty strict limits to the activity of even the cleverest spy.'

'I agree. But this man is not a spy who buys a few wretched subordinates and steals a dozen private letters. He's a genius who has been living as part of our English life. There's nothing he hasn't seen. He's been on terms of intimacy with all kinds of politicians. We know that. He did it as Ivery. They rather liked him, for he was clever and flattered them, and they told him things. But God knows what he saw and heard in his other personalities. For all I know he may have breakfasted at Downing Street with letters of introduction from President Wilson, or visited the Grand Fleet as a distinguished neutral. Then think of the women; how they talk. We're the leakiest society on earth, and we safeguard ourselves by keeping dangerous people out of it. We trust to our outer barrage. But anyone who has really slipped inside has a million chances. And this, remember, is one man in ten millions, a man whose brain never sleeps for a moment, who is quick to seize the slightest hint, who can piece a plan together out of a dozen bits of gossip. It's like - it's as if the Chief of the Intelligence Department were suddenly to desert to the enemy ... The ordinary spy knows only bits of unconnected facts. This man knows our life and our way of thinking and everything about us.'

'Well, but a treatise on English life in time of war won't do much good to the Boche.'

Sir Walter shook his head. 'Don't you realize the explosive stuff that is lying about? Ivery knows enough to make the next German peace offensive really deadly - not the blundering thing which it has been up to now, but something which gets our weak spots on the raw. He knows enough to wreck our campaign in the field. And the awful thing is that we don't know just what he knows or what he is aiming for. This war's a packet of surprises. Both sides are struggling for the margin, the little fraction of advantage, and between evenly matched enemies it's just the extra atom of foreknowledge that tells.'

'Then we've got to push off and get after him,' I said cheerfully.

'But what are you going to do?' asked Macgillivray. 'If it were merely a question of destroying an organization it might be managed, for an organization presents a big front. But it's a question of destroying this one man, and his front is a razor edge. How are you going to find him? It's like looking for a needle in a haystack, and such a needle! A needle which can become a piece of straw or a tin-tack when it chooses!'

'All the same we've got to do it,' I said, remembering old Peter's lesson on fortitude, though I can't say I was feeling very stout-hearted.

Sir Walter flung himself wearily into an arm-chair. 'I wish I could be an optimist,' he said, 'but it looks as if we must own defeat. I've been at this work for twenty years, and, though I've been often beaten, I've always held certain cards in the game. Now I'm hanged if I've any. It looks like a knock-out, Hannay. It's no good deluding ourselves. We're men enough to look facts in the face and tell ourselves the truth. I don't see any ray of light in the business. We've missed our shot by a hairsbreadth and that's the same as missing by miles.'

I remember he looked at Mary as if for confirmation, but she did not smile or nod. Her face was very grave and her eyes looked steadily at him. Then they moved and met mine, and they seemed to give me my marching orders.

'Sir Walter,' I said, 'three years ago you and I sat in this very room. We thought we were done to the world, as we think now. We had just that one miserable little clue to hang on to - a dozen words scribbled in a notebook by a dead man. You thought I was mad when I asked for Scudder's book, but we put our backs into the job and in twenty-four hours we had won out. Remember that then we were fighting against time. Now we have a reasonable amount of leisure. Then we had nothing but a sentence of gibberish. Now we have a great body of knowledge, for Blenkiron has been brooding over Ivery like an old hen, and he knows his ways of working and his breed of confederate. You've got something to work on now. Do you mean to tell me that, when the stakes are so big, you're going to chuck in your hand?'

Macgillivray raised his head. 'We know a good deal about Ivery, but Ivery's dead. We know nothing of the man who was gloriously resurrected this evening in Normandy.'

'Oh, yes we do. There are many faces to the man, but only one mind, and you know plenty about that mind.'

'I wonder,' said Sir Walter. 'How can you know a mind which has no characteristics except that it is wholly and supremely competent? Mere mental powers won't give us a clue. We want to know the character which is behind all the personalities. Above all we want to know its foibles. If we had only a hint of some weakness we might make a plan.'

'Well, let's set down all we know,' I cried, for the more I argued the keener I grew. I told them in some detail the story of the night in the Coolin and what I had heard there.

'There's the two names Chelius and Bommaerts. The man spoke them in the same breath as Effenbein, so they must be associated with Ivery's gang. You've got to get the whole Secret Service of the Allies busy to fit a meaning to these two words. Surely to goodness you'll find something! Remember those names don't belong to the Ivery part, but to the big game behind all the different disguises ... Then there's the talk about the Wild Birds and the Cage Birds. I haven't a guess at what it means. But it refers to some infernal gang, and among your piles of records there must be some clue. You set the intelligence of two hemispheres busy on the job. You've got all the machinery, and it's my experience that if even one solitary man keeps chewing on at a problem he discovers something.'

My enthusiasm was beginning to strike sparks from Macgillivray. He was looking thoughtful now, instead of despondent.

'There might be something in that,' he said, 'but it's a far-out chance.'

'Of course it's a far-out chance, and that's all we're ever going to get from Ivery. But we've taken a bad chance before and won ... Then you've all that you know about Ivery here. Go through his dossier with a small-tooth comb and I'll bet you find something to work on. Blenkiron, you're a man with a cool head. You admit we've a sporting chance.'

'Sure, Dick. He's fixed things so that the lines are across the track, but we'll clear somehow. So far as John S. Blenkiron is concerned he's got just one thing to do in this world, and that's to follow the yellow dog and have him neatly and cleanly tidied up. I've got a stack of personal affronts to settle. I was easy fruit and he hasn't been very respectful. You can count me in, Dick.'

'Then we're agreed,' I cried. 'Well, gentlemen, it's up to you to arrange the first stage. You've some pretty solid staff work to put in before you get on the trail.'

'And you?' Sir Walter asked.

'I'm going back to my brigade. I want a rest and a change. Besides, the first stage is office work, and I'm no use for that. But I'll be waiting to be summoned, and I'll come like a shot as soon as you hoick me out. I've got a presentiment about this thing. I know there'll be a finish and that I'll be in at it, and I think it will be a desperate, bloody business too.'

I found Mary's eyes fixed upon me, and in them I read the same thought. She had not spoken a word, but had sat on the edge of a chair, swinging a foot idly, one hand playing with an ivory fan. She had given me my old orders and I looked to her for confirmation of the new.

'Miss Lamington, you are the wisest of the lot of us. What do you say?'

She smiled - that shy, companionable smile which I had been picturing to myself through all the wanderings of the past month.

'I think you are right. We've a long way to go yet, for the Valley of Humiliation comes only half-way in the Pilgrim's Progress. The next stage was Vanity Fair. I might be of some use there, don't you think?'

I remember the way she laughed and flung back her head like a gallant boy.

'The mistake we've all been making,' she said, 'is that our methods are too terre-a-terre. We've a poet to deal with, a great poet, and we must fling our imaginations forward to catch up with him. His strength is his unexpectedness, you know, and we won't beat him by plodding only. I believe the wildest course is the wisest, for it's the most likely to intersect his ... Who's the poet among us?'

'Peter,' I said. 'But he's pinned down with a game leg in Germany. All the same we must rope him in.'

By this time we had all cheered up, for it is wonderful what a tonic there is in a prospect of action. The butler brought in tea, which it was Bullivant's habit to drink after dinner. To me it seemed fantastic to watch a slip of a girl pouring it out for two grizzled and distinguished servants of the State and one battered soldier - as decorous a family party as you would ask to see - and to reflect that all four were engaged in an enterprise where men's lives must be reckoned at less than thistledown.

After that we went upstairs to a noble Georgian drawing-room and Mary played to us. I don't care two straws for music from an instrument - unless it be the pipes or a regimental band - but I dearly love the human voice. But she would not sing, for singing to her, I fancy, was something that did not come at will, but flowed only like a bird's note when the mood favoured. I did not want it either. I was content to let 'Cherry Ripe' be the one song linked with her in my memory.

It was Macgillivray who brought us back to business.

'I wish to Heaven there was one habit of mind we could definitely attach to him and to no one else.' (At this moment 'He' had only one meaning for us.)

'You can't do nothing with his mind,' Blenkiron drawled. 'You can't loose the bands of Orion, as the Bible says, or hold Leviathan with a hook. I reckoned I could and made a mighty close study of his de-vices. But the darned cuss wouldn't stay put. I thought I had tied him down to the double bluff, and he went and played the triple bluff on me. There's nothing doing that line.'

A memory of Peter recurred to me.

'What about the "blind spot"?' I asked, and I told them old Peter's pet theory. 'Every man that God made has his weak spot somewhere, some flaw in his character which leaves a dull patch in his brain. We've got to find that out, and I think I've made a beginning.' Macgillivray in a sharp voice asked my meaning.

'He's in a funk ... of something. Oh, I don't mean he's a coward. A man in his trade wants the nerve of a buffalo. He could give us all points in courage. What I mean is that he's not clean white all through. There are yellow streaks somewhere in him ... I've given a good deal of thought to this courage business, for I haven't got a great deal of it myself. Not like Peter, I mean. I've got heaps of soft places in me. I'm afraid of being drowned for one thing, or of getting my eyes shot out. Ivery's afraid of bombs - at any rate he's afraid of bombs in a big city. I once read a book which talked about a thing called agoraphobia. Perhaps it's that ... Now if we know that weak spot it helps us in our work. There are some places he won't go to, and there are some things he can't do - not well, anyway. I reckon that's useful.'

'Ye-es,' said Macgillivray. 'Perhaps it's not what you'd call a burning and a shining light.'

'There's another chink in his armour,' I went on. 'There's one person in the world he can never practise his transformations on, and that's me. I shall always know him again, though he appeared as Sir Douglas Haig. I can't explain why, but I've got a feel in my bones about it. I didn't recognize him before, for I thought he was dead, and the nerve in my brain which should have been looking for him wasn't working. But I'm on my guard now, and that nerve's functioning at full power. Whenever and wherever and howsoever we meet again on the face of the earth, it will be "Dr Livingstone, I presume" between him and me.'

'That is better,' said Macgillivray. 'If we have any luck, Hannay, it won't be long till we pull you out of His Majesty's Forces.'

Mary got up from the piano and resumed her old perch on the arm of Sir Walter's chair.

'There's another blind spot which you haven't mentioned.' It was a cool evening, but I noticed that her cheeks had suddenly flushed.

'Last week Mr Ivery asked me to marry him,' she said.