As I watched them Stapleton rose and left the room, while Sir Henry filled his glass again and leaned back in his chair, puffing at his cigar. I heard the creak of a door and the crisp sound of boots upon gravel. The steps passed along the path on the other side of the wall under which I crouched. Looking over, I saw the naturalist pause at the door of an out-house in the corner of the orchard. A key turned in a lock, and as he passed in there was a curious scuffling noise from within. He was only a minute or so inside, and then I heard the key turn once more and he passed me and reentered the house. I saw him rejoin his guest, and I crept quietly back to where my companions were waiting to tell them what I had seen.

“You say, Watson, that the lady is not there?” Holmes asked when I had finished my report.

“No.”

“Where can she be, then, since there is no light in any other room except the kitchen?”

“I cannot think where she is.”

I have said that over the great Grimpen Mire there hung a dense, white fog. It was drifting slowly in our direction and banked itself up like a wall on that side of us, low but thick and well defined. The moon shone on it, and it looked like a great shimmering ice-field, with the heads of the distant tors as rocks borne upon its surface. Holmes’s face was turned towards it, and he muttered impatiently as he watched its sluggish drift.

“It’s moving towards us, Watson.”

“Is that serious?”

“Very serious, indeed - the one thing upon earth which could have disarranged my plans. He can’t be very long, now. It is already ten o’clock. Our success and even his life may depend upon his coming out before the fog is over the path.”

The night was clear and fine above us. The stars shone cold and bright, while a half-moon bathed the whole scene in a soft, uncertain light. Before us lay the dark bulk of the house, its serrated roof and bristling chimneys hard outlined against the silver-spangled sky. Broad bars of golden light from the lower windows stretched across the orchard and the moor. One of them was suddenly shut off. The servants had left the kitchen. There only remained the lamp in the dining-room where the two men, the murderous host and the unconscious guest, still chatted over their cigars.

Every minute that white woolly plain which covered one-half of the moor was drifting closer and closer to the house. Already the first thin wisps of it were curling across the golden square of the lighted window. The farther wall of the orchard was already invisible, and the trees were standing out of a swirl of white vapour. As we watched it the fog-wreaths came crawling round both corners of the house and rolled slowly into one dense bank, on which the upper floor and the roof floated like a strange ship upon a shadowy sea. Holmes struck his hand passionately upon the rock in front of us and stamped his feet in his impatience.

“If he isn’t out in a quarter of an hour the path will be covered. In half an hour we won’t be able to see our hands in front of us.”

“Shall we move farther back upon higher ground?”

“Yes, I think it would be as well.”

So as the fog-bank flowed onward we fell back before it until we were half a mile from the house, and still that dense white sea, with the moon silvering its upper edge, swept slowly and inexorably on.

“We are going too far,” said Holmes. “We dare not take the chance of his being overtaken before he can reach us. At all costs we must hold our ground where we are.” He dropped on his knees and clapped his ear to the ground. “Thank God, I think that I hear him coming.”