"Godbye, old man," called Davies, as the ferry pulled slowly out of Norderney. He looked tired and depressed as he waved goodbye to me, and I was only half awake myself. It was Davies who had woken me, fed me, and packed my bag with motherly care. We had not had time to discuss any further plans, except that he should expect my return on about the 26th, the day after the Germans" meeting.

Böhme was on the ferry too, keeping a close eye on me. However, I managed to get away from him for a few minutes, in order to read Davies's note to me, which he had put into my hand at the last moment. It said:

Their meeting: could be at Norden. It has a station, and high tide there on the 25th will be 10.30 to 11 p. m. Can't be Norddeich, because it has a deep channel for the ferry, so no need to worry about the tide being right.

Other clues: tugs, pilots, depths, railway, Esens, seven of something -- could this mean a land-and-sea defence plan for the North Sea coast?

Sea: there are seven islands from Borkum to Wangeroog (not counting Memmert, which is small), each with its own channel to the mainland. Tugs and pilots needed for finding these channels.

As the day passed, however, I began to feel fresher, and was able to think more clearly. Today was the 23rd, so I had very little time to reach London, find out about Dollmann, and return in time for the meeting on the 25th. I began to imagine how it would be in London -- trying to persuade people in government offices of the urgency of my enquiries. Oh, will you leave a note, sir? Or come back next week to see Mr So-and-So? People are cautious, unwilling to give out information unless they are forced to. And the Navy!

But I was too sleepy to concentrate fully, and I slept for most of the train journey that followed our arrival at Norddeich, only waking up twice. The first time was at Emden, where Böhme and I had to change trains, and I heard someone talking to him about canals. The second was at Leer station, when Böhme woke me to say goodbye. "Don't forget to do your investigating in London!" he cried, smiling falsely.

Land: good railway line running only a few kilometres from the coast, connecting inland villages. Is Esens the centre of this defence plan?

I found a pleasant hotel, and slept for ten hours in a huge, luxurious bed. Early next morning, I was on a train travelling back the way I had come, wearing some old seaman's clothes, which I had bought from a second-hand clothes shop. I was now an ordinary English seaman, going to Emden to join his ship.

Another thought. How sure was I of Davies's safety while I was away? Might they offer to take him to Memmert, put him in a diving-suit, cut off the air -- Stop, that was nonsense! But I had already decided there was no point in returning to London. Instead, I would go as far as Amsterdam, to change my clothes and identity, and sleep just one night in a comfortable bed. Then I would go back to Friesland to look for more clues, and try to solve the riddle.

In Amsterdam, I sent this telegram to my boss:

Very sorry, could not call Norderney for post. Hope extra holiday all right. Please write Hotel du Louvre, Paris.

All day, as I was carried through the Dutch and German countryside, I puzzled over the clues we had gathered. Davies was probably right, and Norden was the place for the meeting on the 25th. "The tide will be right," they had said at Memmert, and high tide was between 10.30 and 11 p. m. My train timetable told me there was a "night train", too, leaving at 7.43 p. m, and stopping at all the villages east of Norden. I determined to be at Norden in time for the meeting, and until then, I would find out all I could about Friesland, and Esens in particular.

In Esens, I walked around in the evening light and discovered several interesting things. A lot of work was being done there, on developing the Bensersiel stream into a canal. I also saw a long, low building, rather like buildings I had seen on Memmert, where barges were being built. I climbed into one of the barges to inspect it more closely, and realized that it was designed not only for canal work, but also for rough water.

At Emden, I bought a ticket to Esens. The train crossed a big canal, which I realized was the Ems-Jade Canal, connecting Emden and Wilhelmshaven, and deep enough to carry gunboats. I looked again at my map of the area. Esens was six kilometres inland from Bensersiel, and a stream ran all the way from Bensersiel to join the Esens-Wittmund Canal. I suddenly remembered the numbers I had heard at Memmert. Perhaps they were the depths of water in canals! The conversation I had heard at Emden station also came back to me, and I felt sure that Böhme was a canal engineer. Was I getting somewhere at last?

The next day was the 25th. In the morning I studied the map again and realized how much I had to explore. There were six more villages like Esens and six more harbours like Bensersiel round the coast, and perhaps these matched Böhme's seven letters, A to G. All seven harbours had channels going through the sands to the open sea, and all seven were connected by a stream or canal to an inland village.

I spent the whole day walking round Friesland. This time I took the identity of a German seaman, and talked to local people, to find out more. I was only suspected of being English once, but that nearly got me into serious trouble. Luckily, however, I managed to get the man drunk and escape from him. But it made me more aware of the danger I was in, so after that I avoided roads and villages, and walked across fields and through streams. Everywhere I went, I discovered that work was going on; all the streams were close to becoming canals, and solid, good quality barges were being built in large numbers.

By now it was midnight, and as there was no one around, I decided to spend the night on the barge. The cold wooden boards brought back the memory of my soft hotel bed, but a spy can't expect luxury every night. At least there was more room to move than on the Dulcibella.

At Esens station, I got out first, and waited in the darkness for the three men to pass. They walked on together, away from the coast and towards the canal. I did not follow them, as I felt sure they would appear at Bensersiel very soon, so I walked quickly to the harbour. There I waited for an uneasy hour, looking at the barges lying at anchor, and wondering if I had guessed wrongly about their plans.

At 7.15 in the evening, I arrived, very tired, at Norden station, for the meeting I hoped would take place there. I was delighted to see von Brüning there already. He did not notice me in my dirty seaman's clothes, and I heard him ask for a ticket to Esens. I bought one, too, and we both got on the 7.43 train. At the last minute, I saw two late arrivals, whom I did not recognize, jump on. A whistle blew, and the train rolled slowly out of the station.

Suddenly I saw the lights of a tug coming into the small harbour. It was Grimm at the helm, with a crew of two seamen. He left them in charge while he jumped on to the shore, and disappeared in the direction of the canal.

Soon after, the sailors hurried back, followed by Grimm. The engine started, and the tug moved slowly away, and then stopped. I heard footsteps from the direction of the canal, and three people jumped on to the deck. We were off again, and this time the tug seemed to be pulling something. What was it? A barge, of course! I had seen one, half-loaded with bricks, lying near the tug in the harbour. (Then I remembered the words from Memmert: "Only one, with half a load.")

I knew I had to get on board somehow -- it was the only way I could follow them -- but how? Fortunately for me, the sailors seemed very interested in the brightly lit pub, and after a short discussion, they tied up the tug, and hurried towards it. This was my chance, and I took it. As soon as they had gone, I ran across the mud, and climbed on board. The only place I could find to hide was the dinghy, which hung over the side of the tug, tied on with thick ropes. I was too excited to be anxious about what might happen if I was discovered, and I hid myself carefully at the bottom of the little boat.

Cautiously, I looked over the edge of the dinghy, and realized we were travelling west, towards Norderney, or Memmert, perhaps. I was perfectly safe, but only until the dinghy was needed. Grimm was steering, and the three passengers were standing at the back of the tug, watching the barge moving smoothly behind us. I recognized tall, bearded von Brüning, and short, fat Böhme, and the third man must be he who "insists on coming". I was almost sure I knew who he was.

Now the tug moved northwards, aiming for the channel between Langeoog and Baltrum. We were taking the open sea route, as the tide was falling, and it would be impossible to cross the sandbanks in the dark. Once through the channel, I expected us to turn west towards Norderney, but we carried on out to sea, where the water was a good deal rougher. After a while, the tug began to turn, and made one complete circle, though for what purpose I had no idea. The behaviour of the three passengers was also puzzling. They spent all the time watching, and clearly talking about, the barge behind us. Finally, we turned west again, in the direction of Memmert.

It was such a daring and clever plan that I found it difficult to believe. But I knew it must be true. Davies and I had never seriously considered that Dollmann and his friends had a plan of attack. we had only ever thought of it as defensive action. But bit by bit, the pieces of the puzzle fell into place, and I had solved the riddle at last.

And then at last I understood. This was the way to England too. What I had just witnessed was a rehearsal for an invasion. This trial trip was designed to show how tugs could pull seagoing barges carrying soldiers and their weapons -- hundreds of barges, carrying thousands of soldiers. This army would gather, not in some great naval harbour, but on an unimportant piece of coast, hidden behind the sandbanks of the Frisian Islands, where nobody would expect an invasion to start from. The barges would be brought down the canals to seven tiny harbours, and when the tide was right, the tugs would pull them through the channels that led between the islands to the open sea. And on to England -- and its undefended east coast.

I was still lost in these thoughts when the tug passed Norderney town and turned south. We were taking the narrow channel to Norddeich, where the passengers would probably land, and I would very likely be discovered in my hiding-place. Somehow I had to escape. Only a kilometre away was the Dulcibella and Davies, if he were safe. What would he do in this situation? The tide was falling, and we were crossing the sandbanks…

A wild idea came to me. I looked quickly at my watch, by the light of a match. It was 2.30 a. m. Low tide would be about 5 o'clock. The tug would be aground until about 7.30 a. m, not in any danger, but safely out of the way.

Grimm was below, in the cabin with his three passengers, while one of the crew was at the helm, with his back to me. Grimm and I were about the same height, and he had left his coat at the top of the ladder down to the cabin. I climbed very quietly out of the dinghy, put on his coat, and pulled my hat down to hide my face. Confidently I walked up to the helmsman, and touched him on the arm, as I had seen Grimm do earlier. The man, used to such commands from his silent captain, moved obediently away.

My plan developed beautifully. I took the helm, and kept the tug in the channel, between the buoys, until I felt the moment had come. Then I suddenly turned the wheel as hard as I could to the right. The seaman shouted a warning, but he was too late. The tug crashed into the sandbank at full speed, and the wheel went stiff in my hands. We were aground.

I think it is safe to say that I was the only one on board who behaved with calmness and common sense in the minutes that followed. Grimm was on deck in seconds, shouting angrily at his crew, and the passengers soon joined in. The wind, darkness and rain made the confusion worse. Unnoticed, I threw off Grimm's coat, and ran back to the dinghy. On the way, I bumped into the unknown passenger -- "he who insists". He thought I was one of Grimm's men, and offered to help me. I saw his face close up, and realized immediately that my earlier suspicion was correct. The leader of the country can, after all, insist on what he likes.

The passenger and I cut the dinghy's ropes, and the boat hit the water with a loud splash. "Lower the boat," I heard Grimm shout, but we had already done it. I jumped in and took up the oars. The wind and tide caught me, and carried me rapidly northwards. I was very quickly out of sight of the barge, and began to row towards Norderney, with the tide. There was an outburst of shouting which soon died away. They would be held tight on the sandbank for at least five hours.