When we reached Brunsbüttel, at the North Sea end of the canal, Davies went off to buy fresh milk. Meanwhile, an official came on board to examine our papers.

We arrived at Kiel at one o'clock in the morning, and the next day we made our preparations for exploring the North Sea sandbanks. We bought food, oil, and plenty of warm clothes, and I wrote to my boss at the Foreign Office, requesting another week or two of holiday. I asked him to send his reply to me at the post office in Norderney, as Davies and I had no fixed address. When we were ready, we sailed the Dulcibella through the huge iron gates of the ship canal that connects Germany's two seas, the North Sea and the Baltic. For two days, with some of the largest ships in the world, we travelled slowly through the great waterway, wondering at the engineering and organizational ability of the nation that had created it.

"Dulcibella," he said. "Someone from a big yacht was asking about you the day before yesterday."

"Did he say what he wanted?" I asked.

"Did she say where?" I asked.

"No, she was going into the North Sea."

"Not 'he', Captain. It was a young lady," replied the official. He smiled knowingly. "She wanted to know if you had gone through the canal. It's a pity you missed her."

"Well, we'll probably see her in Hamburg," I said.

With the strong tide, we soon reached Cuxhaven. Davies's plan was to explore the sandbanks between the Scharhörn and Cuxhaven, to find the channels and put them on his charts. Soon I realized that banks of yellow and brown sand were appearing to the west of us. Davies was looking delighted, as we left the main channel and sailed westwards, straight for the sandbanks.

"No," he answered. "But don't worry, Captain. There are plenty of pretty girls in Hamburg." He laughed, and went on to the next boat, as Davies returned with the milk.

Then the great gates opened, and we turned the Dulcibella towards the North Sea.

"Well, Davies," I said, when I told him what the official had said, "Dollmann can't believe he's got rid of you, if he sent his daughter to enquire after you like that."

"I don't think he sent her to ask," said Davies. "I think it was her own idea to find out." He had a strange look on his face, half happy, half confused.

We were in an extremely narrow channel. While I measured the depth of the water with the lead line, and called out the metres, Davies steered, pulled in the sails, and kept an eye on the chart. But in spite of our carefulness, there was a bumping sound from below, and we ran aground.

"Centreboard up," he cried. "Now for some real sailing!"

When we went back on deck, it was low tide, and the yacht was sitting on the top of a sandbank. As far as the eye could see, there was nothing but sand, broken here and there by the winding path of a channel. Some of these channels still held water, but others had dried out completely. Under a dull grey sky, the wind blew across this wide empty space, crying softly like a child in pain. It seemed the saddest, loneliest place on earth.

"There's nothing to worry about," Davies said cheerfully. "When the tide rises, we'll float off the sandbank. This is a good moment to have lunch."

Davies, however, had climbed the mast and was examining the sand enthusiastically. His face shone with pleasure. I had never seen him look happier.

We raised the sail and set off, following the channel we had just explored. All around us stretched the sea, with nothing to show where the channel was. I felt completely lost, but Davies was his usual confident self. He made me call out the depth of water every few seconds, while he steered carefully, feeling for the edge of the sandbank.

"There!" he cried. "You see what I mean? Have you ever seen anything like this?" He climbed down and then jumped down on to the sandbank. "Come on!" he called. "The only way to understand a place like this is to explore it at low tide."

"Right!" he said when he had finished. "Now let's get back to the Dulcibella. This tide's rising fast."

I joined him, and together we ran over the sandbanks as fast as our heavy rubber boots would let us. Davies noted down every post or marker, and every bend in the channels, on his chart.

We turned, and ran for the yacht. I was thankful to reach it in time, before the sands were completely covered by the incoming tide. I stood on deck, and watched the sea making its way across the desert of sand. Under my feet the Dulcibella gave a jump, paused, gave another jump, and was suddenly floating again on the grey waters, which now hid the wide area of sand where Davies and I had just walked.

"Right! Let go the anchor," he said at last, "and lower the sails."

And sure enough, we could hear the waves crashing on the sandbank. As darkness fell, the wind grew fiercer, and the sea rougher. This was the first time we had anchored at night out of sight of land, and I found it a frightening experience as the Dulcibella rolled from side to side and the sea thundered on the sandbank. But I knew now that Davies was a yachtsman of extraordinary skill, and if he said that we were perfectly safe, then safe we were.

It was getting dark. The German coast had already disappeared, and the sea all around us looked exactly the same to me. But I knew that Davies carried a picture in his head of the complicated pattern of the sands around and beneath us, which he had already charted.

"Where are we?" I asked.

Nothing happened in the next ten days to disturb us at our work. We spent all the daylight hours exploring the sandbanks and channels around Cuxhaven, and marking the changes on the charts. We found nothing to explain why Dollmann wanted to get rid of Davies, and nobody asked questions or tried to stop us.

"In the shelter of the Hohenhörn, in the channel where Bartels took me that night. If you listen, you can hear the waves breaking on the Hohenhörn, where I went aground."

So we left the Cuxhaven area, and set sail for the Frisian Islands. Helped by a strong easterly wind, we sailed all the way without stopping. We reached the island of Wangeroog just before dark, and ran the Dulcibella aground on the sand a hundred metres from the shore. There were three fishing boats anchored about a kilometre from us. Davies set out on foot across the sand to get fresh water and oil, leaving me alone on the yacht.

I was wide awake in a second, and sat up and listened. The call came again, "Hello there, on the yacht!"

"I'm sure it's something to do with these channels through the sands," said Davies at last. "But nobody seems to mind our being here. The answer to the riddle must be near where I first met Dollmann, at Norderney."

"Make sure you keep the light burning on the mast," he said as he left. "It's my only guide back to the yacht."

It had been an exhausting day and I was feeling very tired, so I lay down on the seat in the cabin. I was half asleep when I heard footsteps on the sand outside, and then a voice calling in German, "Hello there, on the yacht!"

"I don't think the local Germans would steal from a yacht," replied Davies. "And they wouldn't put out the mast light. They're all seamen and know how important it is."

I did not reply. Was this, I wondered, something to do with the mystery? I heard someone climb onto the deck of the Dulcibella. Suddenly the light on the mast went out. The visitor walked along the deck to the doorway, and began to climb down the ladder into the cabin. I should have waited until he was down, but I was too eager to catch him. I jumped towards the ladder and caught hold of a leg. My unknown visitor kicked out, pulled himself free, and reached the deck, leaving me holding a boot. I rushed up on deck after him, but he was too fast for me, and I did not manage to catch him. I thought of Davies coming back with the oil and water, unable to find the yacht, so I quickly relit the mast light.

When Davies returned, I told him about our visitor.

"I think we're being watched," I said, "unless he was just a thief, from one of those fishing boats, perhaps."

We sat up late looking at the chart. There were seven islands in the German Frisian group, separated from the coast by the sands, which were mostly dry at low tide. There were small villages, just a few houses and a church, on most of the islands. Norderney had the only harbour. It was quite a busy little seaside town in the summer, but it was almost empty for the rest of the year. The mainland had no large towns either, just a few small villages. Davies pointed to the row of islands on the chart. "Just look at the fine, sheltered harbour hidden between these islands and the coast! It's fifty kilometres long and ten kilometres wide, perfect for small gunboats."

We discussed what to do. The charts we were using, with all our corrections and notes, were the only things anyone might want to see, so we decided to hide them and the log-book. Now if anyone came on board, they would think we were just two harmless young men, on an autumn sailing holiday.

"Have you noticed," I asked, "that on the mainland there's a channel or a stream leading to each of those villages on the coast? Shouldn't we explore those too?"

"I don't think so," he replied. "They only lead to those tiny villages." Davies hated spending time on land.

"Perhaps we should have a look at them," I said.

"Yes, but look," I said, pointing to the chart, "there are small harbours at Bensersiel and other villages."

We spent the next day exploring the channels around Wangeroog. There was only one of the fishing boats still there, and as we passed, I saw it was called the Kormoran. Later in the afternoon, we caught sight of a small grey naval ship, moving slowly past, outside the islands.

"So there are," said Davies. "They're probably just big enough for the local fishing boats."

"Perhaps," he agreed unwillingly. "But there's a lot more real work to be done further out, among the islands."

"The Blitz," said Davies. "Von Brüning's ship."

As it happened, we went to Bensersiel sooner than we had expected. Two days later a strong south-west wind began to blow, promising storms to follow. We saw that the Blitz had already taken shelter, and was now anchored just south of Spiekeroog, next to several fishing boats. During the afternoon, the wind went round to the north-east -- a bad sign, Davies said -- and the sky turned black, bringing violent rainstorms. We decided to shelter at Bensersiel and had an exciting run through the sandbanks and a very rough sea to the shore, where with great skill Davies managed to bring us safely into the tiny harbour. He, of course, always preferred to anchor for the night away from the shore, but I was glad to step on to dry land, after more than two weeks at sea.