I saw an old green felt hat, and below it lean tweed-clad shoulders. Then I saw a knapsack with a stick slung through it, as the owner wriggled his way on to a shelf. Presently he turned his face upward to judge the remaining distance. It was the face of a young man, a face sallow and angular, but now a little flushed with the day's sun and the work of climbing. It was a face that I had first seen at Fosse Manor.

I felt suddenly sick and heartsore. I don't know why, but I had never really associated the intellectuals of Biggleswick with a business like this. None of them but Ivery, and he was different. They had been silly and priggish, but no more - I would have taken my oath on it. Yet here was one of them engaged in black treason against his native land. Something began to beat in my temples when I remembered that Mary and this man had been friends, that he had held her hand, and called her by her Christian name. My first impulse was to wait till he got up and then pitch him down among the boulders and let his German accomplices puzzle over his broken neck.

With difficulty I kept down that tide of fury. I had my duty to do, and to keep on terms with this man was part of it. I had to convince him that I was an accomplice, and that might not be easy. I leaned over the edge, and, as he got to his feet on the ledge above the boiler-plates, I whistled so that he turned his face to me. 'Hullo, Wake,'I said.

He started, stared for a second, and recognized me. He did not seem over-pleased to see me.

'Brand!' he cried. 'How did you get here?'

He swung himself up beside me, straightened his back and unbuckled his knapsack. 'I thought this was my own private sanctuary, and that nobody knew it but me. Have you spotted the cave? It's the best bedroom in Skye.' His tone was, as usual, rather acid.

That little hammer was beating in my head. I longed to get my hands on his throat and choke the smug treason in him. But I kept my mind fixed on one purpose - to persuade him that I shared his secret and was on his side. His off-hand self-possession seemed only the clever screen of the surprised conspirator who was hunting for a plan.

We entered the cave, and he flung his pack into a corner. 'Last time I was here,' he said, 'I covered the floor with heather. We must get some more if we would sleep soft.' In the twilight he was a dim figure, but he seemed a new man from the one I had last seen in the Moot Hall at Biggleswick. There was a wiry vigour in his body and a purpose in his face. What a fool I had been to set him down as no more than a conceited fidneur!

He went out to the shelf again and sniffed the fresh evening. There was a wonderful red sky in the west, but in the crevice the shades had fallen, and only the bright patches at either end told of the sunset. 'Wake,' I said, 'you and I have to understand each other. I'm a friend of Ivery and I know the meaning of this place. I discovered it by accident, but I want you to know that I'm heart and soul with you. You may trust me in tonight's job as if I were Ivery himself.'

He swung round and looked at me sharply. His eyes were hot again, as I remembered them at our first meeting.

'What do you mean? How much do you know?'

The hammer was going hard in my forehead, and I had to pull myself together to answer.

'I know that at the end of this crack a message was left last night, and that someone came out of the sea and picked it up. That someone is coming again when darkness falls, and there will be another message.'

He had turned his head away. 'You are talking nonsense. No submarine could land on this coast.'

I could see that he was trying me. 'This morning,' I said, 'I swam in the deep-water inlet below us. It is the most perfect submarine shelter in Britain.'

He still kept his face from me, looking the way he had come. For a moment he was silent, and then he spoke in the bitter, drawling voice which had annoyed me at Fosse Manor.

'How do you reconcile this business with your principles, Mr Brand? You were always a patriot, I remember, though you didn't see eye to eye with the Government.'

It was not quite what I expected and I was unready. I stammered in my reply. 'It's because I am a patriot that I want peace. I think that ... I mean ...'

'Therefore you are willing to help the enemy to win?'

'They have already won. I want that recognized and the end hurried on.' I was getting my mind clearer and continued fluently. 'The longer the war lasts, the worse this country is ruined. We must make the people realize the truth, and -'

But he swung round suddenly, his eyes blazing.

'You blackguard!' he cried, 'you damnable blackguard!' And he flung himself on me like a wild-cat.

I had got my answer. He did not believe me, he knew me for a spy, and he was determined to do me in. We were beyond finesse now, and back at the old barbaric game. It was his life or mine. The hammer beat furiously in my head as we closed, and a fierce satisfaction rose in my heart. He never had a chance, for though he was in good trim and had the light, wiry figure of the mountaineer, he hadn't a quarter of my muscular strength. Besides, he was wrongly placed, for he had the outside station. Had he been on the inside he might have toppled me over the edge by his sudden assault. As it was, I grappled him and forced him to the ground, squeezing the breath out of his body in the process. I must have hurt him considerably, but he never gave a cry. With a good deal of trouble I lashed his hands behind his back with the belt of my waterproof, carried him inside the cave and laid him in the dark end of it. Then I tied his feet with the strap of his own knapsack. I would have to gag him, but that could wait.

I had still to contrive a plan of action for the night, for I did not know what part he had been meant to play in it. He might be the messenger instead of the Portuguese Jew, in which case he would have papers about his person. If he knew of the cave, others might have the same knowledge, and I had better shift him before they came. I looked at my wrist-watch, and the luminous dial showed that the hour was half past nine.

Then I noticed that the bundle in the corner was sobbing. It was a horrid sound and it worried me. I had a little pocket electric torch and I flashed it on Wake's face. If he was crying, it was with dry eyes.

'What are you going to do with me?' he asked.

'That depends,' I said grimly.

'Well, I'm ready. I may be a poor creature, but I'm damned if I'm afraid of you, or anything like you.' That was a brave thing to say, for it was a lie; his teeth were chattering.

'I'm ready for a deal,' I said.

'You won't get it,' was his answer. 'Cut my throat if you mean to, but for God's sake don't insult me ... I choke when I think about you. You come to us and we welcome you, and receive you in our houses, and tell you our inmost thoughts, and all the time you're a bloody traitor. You want to sell us to Germany. You may win now, but by God! your time will come! That is my last word to you ... you swine!'

The hammer stopped beating in my head. I saw myself suddenly as a blind, preposterous fool. I strode over to Wake, and he shut his eyes as if he expected a blow. Instead I unbuckled the straps which held his legs and arms.

'Wake, old fellow,' I said, 'I'm the worst kind of idiot. I'll eat all the dirt you want. I'll give you leave to knock me black and blue, and I won't lift a hand. But not now. Now we've another job on hand. Man, we're on the same side and I never knew it. It's too bad a case for apologies, but if it's any consolation to you I feel the lowest dog in Europe at this moment.'

He was sitting up rubbing his bruised shoulders. 'What do you mean?' he asked hoarsely.

'I mean that you and I are allies. My name's not Brand. I'm a soldier - a general, if you want to know. I went to Biggleswick under orders, and I came chasing up here on the same job. Ivery's the biggest German agent in Britain and I'm after him. I've struck his communication lines, and this very night, please God, we'll get the last clue to the riddle. Do you hear? We're in this business together, and you've got to lend a hand.'

I told him briefly the story of Gresson, and how I had tracked his man here. As I talked we ate our supper, and I wish I could have watched Wake's face. He asked questions, for he wasn't convinced in a hurry. I think it was my mention of Mary Lamington that did the trick. I don't know why, but that seemed to satisfy him. But he wasn't going to give himself away.

'You may count on me,' he said, 'for this is black, blackguardly treason. But you know my politics, and I don't change them for this. I'm more against your accursed war than ever, now that I know what war involves.'

'Right-o,' I said, 'I'm a pacifist myself. You won't get any heroics about war from me. I'm all for peace, but we've got to down those devils first.'

It wasn't safe for either of us to stick in that cave, so we cleared away the marks of our occupation, and hid our packs in a deep crevice on the rock. Wake announced his intention of climbing the tower, while there was still a faint afterglow of light. 'It's broad on the top, and I can keep a watch out to sea if any light shows. I've been up it before. I found the way two years ago. No, I won't fall asleep and tumble off. I slept most of the afternoon on the top of Sgurr Vhiconnich, and I'm as wakeful as a bat now.'

I watched him shin up the face of the tower, and admired greatly the speed and neatness with which he climbed. Then I followed the crevice southward to the hollow just below the platform where I had found the footmarks. There was a big boulder there, which partly shut off the view of it from the direction of our cave. The place was perfect for my purpose, for between the boulder and the wall of the tower was a narrow gap, through which I could hear all that passed on the platform. I found a stance where I could rest in comfort and keep an eye through the crack on what happened beyond.

There was still a faint light on the platform, but soon that disappeared and black darkness settled down on the hills. It was the dark of the moon, and, as had happened the night before, a thin wrack blew over the sky, hiding the stars. The place was very still, though now and then would come the cry of a bird from the crags that beetled above me, and from the shore the pipe of a tern or oyster-catcher. An owl hooted from somewhere up on the tower. That I reckoned was Wake, so I hooted back and was answered. I unbuckled my wrist-watch and pocketed it, lest its luminous dial should betray me; and I noticed that the hour was close on eleven. I had already removed my shoes, and my jacket was buttoned at the collar so as to show no shirt. I did not think that the coming visitor would trouble to explore the crevice beyond the platform, but I wanted to be prepared for emergencies.

Then followed an hour of waiting. I felt wonderfully cheered and exhilarated, for Wake had restored my confidence in human nature. In that eerie place we were wrapped round with mystery like a fog. Some unknown figure was coming out of the sea, the emissary of that Power we had been at grips with for three years. It was as if the war had just made contact with our own shores, and never, not even when I was alone in the South German forest, had I felt so much the sport of a whimsical fate. I only wished Peter could have been with me. And so my thoughts fled to Peter in his prison camp, and I longed for another sight of my old friend as a girl longs for her lover.

Then I heard the hoot of an owl, and presently the sound of careful steps fell on my ear. I could see nothing, but I guessed it was the Portuguese Jew, for I could hear the grinding of heavily nailed boots on the gritty rock.

The figure was very quiet. It appeared to be sitting down, and then it rose and fumbled with the wall of the tower just beyond the boulder behind which I sheltered. It seemed to move a stone and to replace it. After that came silence, and then once more the hoot of an owl. There were steps on the rock staircase, the steps of a man who did not know the road well and stumbled a little. Also they were the steps of one without nails in his boots.

They reached the platform and someone spoke. It was the Portuguese Jew and he spoke in good German.

'Die vogelein schweigen im Walde,' he said.

The answer came from a clear, authoritative voice.

'Warte nur, balde ruhest du auch.'

Clearly some kind of password, for sane men don't talk about little birds in that kind of situation. It sounded to me like indifferent poetry.

Then followed a conversation in low tones, of which I only caught odd phrases. I heard two names - Chelius and what sounded like a Dutch word, Bommaerts. Then to my joy I caught Effenbein, and when uttered it seemed to be followed by a laugh. I heard too a phrase several times repeated, which seemed to me to be pure gibberish - Die Stubenvogel verstehn. It was spoken by the man from the sea. And then the word Wildvogel. The pair seemed demented about birds.

For a second an electric torch was flashed in the shelter of the rock, and I could see a tanned, bearded face looking at some papers. The light disappeared, and again the Portuguese Jew was fumbling with the stones at the base of the tower. To my joy he was close to my crack, and I could hear every word. 'You cannot come here very often,' he said, 'and it may be hard to arrange a meeting. See, therefore, the place I have made to put the Viageffutter. When I get a chance I will come here, and you will come also when you are able. Often there will be nothing, but sometimes there will be much.'

My luck was clearly in, and my exultation made me careless. A stone, on which a foot rested, slipped and though I checked myself at once, the confounded thing rolled down into the hollow, making a great clatter. I plastered myself in the embrasure of the rock and waited with a beating heart. The place was pitch dark, but they had an electric torch, and if they once flashed it on me I was gone. I heard them leave the platform and climb down into the hollow. There they stood listening, while I held my breath. Then I heard 'Nix, mein freund,' and the two went back, the naval officer's boots slipping on the gravel.

They did not leave the platform together. The man from the sea bade a short farewell to the Portuguese Jew, listening, I thought, impatiently to his final message as if eager to be gone. It was a good half-hour before the latter took himself off, and I heard the sound of his nailed boots die away as he reached the heather of the moor.

I waited a little longer, and then crawled back to the cave. The owl hooted, and presently Wake descended lightly beside me; he must have known every foothold and handhold by heart to do the job in that inky blackness. I remember that he asked no question of me, but he used language rare on the lips of conscientious objectors about the men who had lately been in the crevice. We, who four hours earlier had been at death grips, now curled up on the hard floor like two tired dogs, and fell sound asleep.

I woke to find Wake in a thundering bad temper. The thing he remembered most about the night before was our scrap and the gross way I had insulted him. I didn't blame him, for if any man had taken me for a German spy I would have been out for his blood, and it was no good explaining that he had given me grounds for suspicion. He was as touchy about his blessed principles as an old maid about her age. I was feeling rather extra buckish myself and that didn't improve matters. His face was like a gargoyle as we went down to the beach to bathe, so I held my tongue. He was chewing the cud of his wounded pride.

But the salt water cleared out the dregs of his distemper. You couldn't be peevish swimming in that jolly, shining sea. We raced each other away beyond the inlet to the outer water, which a brisk morning breeze was curling. Then back to a promontory of heather, where the first beams of the sun coming over the Coolin dried our skins. He sat hunched up staring at the mountains while I prospected the rocks at the edge. Out in the Minch two destroyers were hurrying southward, and I wondered where in that waste of blue was the craft which had come here in the night watches.

I found the spoor of the man from the sea quite fresh on a patch of gravel above the tide-mark.

'There's our friend of the night,' I said.

'I believe the whole thing was a whimsy,' said Wake, his eyes on the chimneys of Sgurr Dearg. 'They were only two natives - poachers, perhaps, or tinkers.'

'They don't speak German in these parts.'

'It was Gaelic probably.'

'What do you make of this, then?' and I quoted the stuff about birds with which they had greeted each other.

Wake looked interested. 'That's Uber allen Gipfeln. Have you ever read Goethe?'

'Never a word. And what do you make of that?' I pointed to a flat rock below tide-mark covered with a tangle of seaweed. It was of a softer stone than the hard stuff in the hills and somebody had scraped off half the seaweed and a slice of the side. 'That wasn't done yesterday morning, for I had my bath here.'

Wake got up and examined the place. He nosed about in the crannies of the rocks lining the inlet, and got into the water again to explore better. When he joined me he was smiling. 'I apologize for my scepticism,' he said. 'There's been some petrol-driven craft here in the night. I can smell it, for I've a nose like a retriever. I daresay you're on the right track. Anyhow, though you seem to know a bit about German, you could scarcely invent immortal poetry.'

We took our belongings to a green crook of the burn, and made a very good breakfast. Wake had nothing in his pack but plasmon biscuits and raisins, for that, he said, was his mountaineering provender, but he was not averse to sampling my tinned stuff. He was a different-sized fellow out in the hills from the anaemic intellectual of Biggleswick. He had forgotten his beastly self-consciousness, and spoke of his hobby with a serious passion. It seemed he had scrambled about everywhere in Europe, from the Caucasus to the Pyrenees. I could see he must be good at the job, for he didn't brag of his exploits. It was the mountains that he loved, not wriggling his body up hard places. The Coolin, he said, were his favourites, for on some of them you could get two thousand feet of good rock. We got our glasses on the face of Sgurr Alasdair, and he sketched out for me various ways of getting to its grim summit. The Coolin and the Dolomites for him, for he had grown tired of the Chamonix aiguilles. I remember he described with tremendous gusto the joys of early dawn in Tyrol, when you ascended through acres of flowery meadows to a tooth of clean white limestone against a clean blue sky. He spoke, too, of the little wild hills in the Bavarian Wettersteingebirge, and of a guide he had picked up there and trained to the job.

'They called him Sebastian Buchwieser. He was the jolliest boy you ever saw, and as clever on crags as a chamois. He is probably dead by now, dead in a filthy jaeger battalion. That's you and your accursed war.'

'Well, we've got to get busy and end it in the right way,' I said. 'And you've got to help, my lad.'

He was a good draughtsman, and with his assistance I drew a rough map of the crevice where we had roosted for the night, giving its bearings carefully in relation to the burn and the sea. Then I wrote down all the details about Gresson and the Portuguese Jew, and described the latter in minute detail. I described, too, most precisely the cache where it had been arranged that the messages should be placed. That finished my stock of paper, and I left the record of the oddments overheard of the conversation for a later time. I put the thing in an old leather cigarette-case I possessed, and handed it to Wake. 'You've got to go straight off to the Kyle and not waste any time on the way. Nobody suspects you, so you can travel any road you please. When you get there you ask for Mr Andrew Amos, who has some Government job in the neighbourhood. Give him that paper from me. He'll know what to do with it all right. Tell him I'll get somehow to the Kyle before midday the day after tomorrow. I must cover my tracks a bit, so I can't come with you, and I want that thing in his hands just as fast as your legs will take you. If anyone tries to steal it from you, for God's sake eat it. You can see for yourself that it's devilish important.'

'I shall be back in England in three days,' he said. 'Any message for your other friends?'

'Forget all about me. You never saw me here. I'm still Brand, the amiable colonial studying social movements. If you meet Ivery, say you heard of me on the Clyde, deep in sedition. But if you see Miss Lamington you can tell her I'm past the Hill Difficulty. I'm coming back as soon as God will let me, and I'm going to drop right into the Biggleswick push. Only this time I'll be a little more advanced in my views ... You needn't get cross. I'm not saying anything against your principles. The main point is that we both hate dirty treason.'

He put the case in his waistcoat pocket. 'I'll go round Garsbheinn,' he said, 'and over by Camasunary. I'll be at the Kyle long before evening. I meant anyhow to sleep at Broadford tonight ... Goodbye, Brand, for I've forgotten your proper name. You're not a bad fellow, but you've landed me in melodrama for the first time in my sober existence. I have a grudge against you for mixing up the Coolin with a shilling shocker. You've spoiled their sanctity.'

'You've the wrong notion of romance,' I said. 'Why, man, last night for an hour you were in the front line - the place where the enemy forces touch our own. You were over the top - you were in No-man's-land.'

He laughed. 'That is one way to look at it'; and then he stalked off and I watched his lean figure till it was round the turn of the hill.

All that morning I smoked peacefully by the burn, and let my thoughts wander over the whole business. I had got precisely what Blenkiron wanted, a post office for the enemy. It would need careful handling, but I could see the juiciest lies passing that way to the Grosses Haupiquartier. Yet I had an ugly feeling at the back of my head that it had been all too easy, and that Ivery was not the man to be duped in this way for long. That set me thinking about the queer talk on the crevice. The poetry stuff I dismissed as the ordinary password, probably changed every time. But who were Chelius and Bommaerts, and what in the name of goodness were the Wild Birds and the Cage Birds? Twice in the past three years I had had two such riddles to solve - Scudder's scribble in his pocket- book, and Harry Bullivant's three words. I remembered how it had only been by constant chewing at them that I had got a sort of meaning, and I wondered if fate would some day expound this puzzle also.

Meantime I had to get back to London as inconspicuously as I had come. It might take some doing, for the police who had been active in Morvern might be still on the track, and it was essential that I should keep out of trouble and give no hint to Gresson and his friends that I had been so far north. However, that was for Amos to advise me on, and about noon I picked up my waterproof with its bursting pockets and set off on a long detour up the coast. All that blessed day I scarcely met a soul. I passed a distillery which seemed to have quit business, and in the evening came to a little town on the sea where I had a bed and supper in a superior kind of public-house.

Next day I struck southward along the coast, and had two experiences of interest. I had a good look at Ranna, and observed that the Tobermory was no longer there. Gresson had only waited to get his job finished; he could probably twist the old captain any way he wanted. The second was that at the door of a village smithy I saw the back of the Portuguese Jew. He was talking Gaelic this time - good Gaelic it sounded, and in that knot of idlers he would have passed for the ordinariest kind of gillie.

He did not see me, and I had no desire to give him the chance, for I had an odd feeling that the day might come when it would be good for us to meet as strangers.

That night I put up boldly in the inn at Broadford, where they fed me nobly on fresh sea-trout and I first tasted an excellent liqueur made of honey and whisky. Next morning I was early afoot, and well before midday was in sight of the narrows of the Kyle, and the two little stone clachans which face each other across the strip of sea. About two miles from the place at a turn of the road I came upon a farmer's gig, drawn up by the wayside, with the horse cropping the moorland grass. A man sat on the bank smoking, with his left arm hooked in the reins. He was an oldish man, with a short, square figure, and a woollen comforter enveloped his throat.