“We must keep together. It may be firm going here but it would be well not to get separated where the sands are more perilous.”

So we walked on until we were, at the very least, level with the beacon of the church tower. It seemed far away now across the flooding sands.

“We must take care that the tide does not get behind us,” I said a little breathlessly.

Holmes was not listening.

“Now stop a bit,” he said, “Let us have our bearings. We are the Chastelnau brothers. Here it is that either you or I kill the other. The killer may have carried out a long-prepared plan. Alternatively, it may have been provocation, a sudden heat and a terrifying accident. In either event, what would the survivor do next?”

“Get back to the Old Light! Where else should he go?”

“Very well, Watson. You have committed murder or, at the least manslaughter. Now, pray lead on.”

This was not what I had bargained for but I was relieved to be turning back before the flood tide encroached any further. As a soldier, I was not unprepared for the challenge. There was no light along the western horizon nor a moon in the sky. The lamps of the village were scarcely pinpricks. I heard an insistent murmuring from the dark billowing sea which was a good deal louder now than when we had set out. For the first time, I noticed a sharp north-east wind gathering at our backs, a light spot or two of cold rain. It was a reminder that, despite the pleasant sunlight of that afternoon, this was the season of equinoctial storms.

When I had joined the Army Medical Department a decade earlier and sailed for Afghanistan with the Northumberland Fusiliers, even a surgeon’s training for service overseas included a course of instruction in map-reading, compass-bearings, judging distances and identifying terrain. In my mind I now constructed a square map. In the top right-hand corner was the beacon of the church tower. In the top left-hand corner was the Old Light. Along the bottom was the line of the incoming tide. Holmes and I were in the bottom right-hand corner, walking parallel with the foot of the map.

We were following what I had judged to be a rib of sand six inches or more above the level of the dark beach around us and therefore firmer. The temptation was to cut a diagonal across the square map towards the Old Light. Fortunately, I had surveyed the terrain from the windows of the barrack-room that afternoon and had seen that such a diagonal would take us into lower ground, probably already flooded by the tide and possibly containing quicksand. The prudent line of march was still to follow the bottom line of the map until we were face-to-face with the lighthouse beam. Then we should know that we had reached the bottom left-hand corner and need only take a right-angle and walk straight into the beam to reach the safety of the iron ladder to the barrack-room.

The sand beneath our feet was softer but there was no doubt that we were still on slightly higher ground. I thought of Holmes’s question and my answer. Suppose I were my brother’s murderer, making my escape. In the first place, it was impossible that I should go anywhere but the Old Light. I assumed that I would not have intended murder when I set out. Therefore I would not have been prepared for immediate flight without returning to the barrack-room.

Holmes said nothing in all this time but appeared content to follow where I led. The light of St Clement’s beacon was dropping away behind us on our right. The beam from the Old Light was ahead but shining at an angle, slightly away from us. We seemed in danger of pulling inland behind it. That must be avoided at all costs for it was where I had noticed earlier that the mudflats lay and the treacherous “shivering sands” might be. It was easy enough to set a course a little further to our left. This brought us slightly closer to the tide but also took us further round on an angle that should put us in the lighthouse beam. Once there, we were safe.

I had begun to feel that a man could make too much of such difficulties as this beach had presented. Then I put my right foot forward and, before I could pull back, felt the leg sink half way up the shin in freezing mud.

“Stop! Stop, Holmes!”

We were almost level with the lighthouse beam but were somehow in the very terrain I had tried to avoid. To one side, among what was now fathomless mud rather than firm sand, I could see strands of grass, limp and wet. Fresh water could mean only the river, which we should not have encountered at all. Though we were still short of the lighthouse beam where it crossed our path, we had somehow come too far. How could that be? How could we be where we evidently were-and how had we got there? The doomed baggage-train of King John, all those centuries ago, had thought themselves secure. Had we made an error in common with them? I had dipped into the chronicles which Holmes had brought and now recalled with dismay a warning that the first step to destruction in shifting sands and estuaries is when the victims lose their way-as we had done now. The ground opened in the midst of the waters, wrote the medieval chronicler Roger of Wendover, and whirlpools sucked down men and horses. It was already too late. To those who might help us, our cries would be inaudible.

The true terror, in the darkness and the fast sinking mud, is to know that one has followed meticulously the path to safety but come only to the verge of a cold and cruel death. There was no explanation and, without that, we were done for. We must move instantly or the incoming tide would overwhelm us-but where was our path? There was nothing but the surf and the sinking sand around us, on every side.

By every calculation, we should be on a firm river-bank path. It must by any logic lead us directly and easily back to the Old Light and the village, both of them now lost in the mist. The intermittent beam which must be our salvation was drawing us ever deeper into the deadly chill of the river mud. This was surely impossible, which was only to say that we were utterly lost in the darkness and the sea fog. But now there was a quiet and sinister ripple of incoming water across the path behind us and nothing but the filling estuary about our legs in front.

Where, by all reason, there should have been firm ground, the softening mud offered no foothold but a fathomless trap.

I hope I should not have fallen into a panic but the cliché of not knowing which way to turn was never more true. The square map firmly in my head was now submerged as surely as the drowning Roland Chastelnau.

“I think,” said Sherlock Holmes, “you had better leave this to me after all.”

I was never more glad to do so. In that familiar voice there was assurance and a confidence which I was far from feeling.

“Step back!” he said.

His hand gripped my upper arm, as I performed a grotesque about-turn with my left leg, relying on his support. The last of the firm ground, from which I had stepped a moment ago, was under my right foot once more. I saw by the reflected light of the beacon that he was probing the mud with his walking stick. It was a painstaking business but after a dozen carefully chosen steps the fearful sense of bottomless silt became something which yielded a little but then held firm. But now we were walking away from the beam of the Old Light-surely in the wrong direction?

“A little further still I think, if my calculations are correct,” Holmes remarked casually.

What those calculations were, I could not say. By mine, we were heading straight into the shivering sands. Yet the mud underfoot was still turning to firmer ground. Presently, at each step that I took, the slush did no worse than engulf the lower portion of my boots. And yet the Old Light seemed to fall further behind us. This was no time to argue the matter, I took one more step and felt what seemed like compact earth under my boot.