The passage seemed to run down one side of the building, but the vaults that gave on to it led each one into another, so that the place was something of a labyrinth. The knocking sounded distinctly now, echoing through the empty cellars. Charles held his torch lowered, so that the circle of light was thrown barely a yard in front of him.
Suddenly the knocking ceased, and at once both men stood still, waiting for some sound to guide them.
Ahead of them, where the passage ended, something moved. Charles flashed his torch upwards, and for a brief instant he and Peter caught a glimpse of a vague figure. Then, as though it had melted into the wall, it was gone, and a wail as of a soul in torment seemed to fill the entire place.
The sweat broke out on both men's foreheads, and for a second neither could move for sheer horror. Then Charles pulled himself together and dashed forward, shouting to Peter to follow.
"My God, what was it?" Peter gasped.
"The groan we've all heard, of course. Damn it, he can't have got away!"
But the place where the figure had stood was quite empty. An embrasure in the wall seemed to mark the spot where they had seen it, yet if the apparent melting into the wall had been no more than a drawing back into this niche that could not solve the complete disappearance of the figure.
The two men stared at one another. Charles passed the back of his hand across his forehead. "But - but I saw it!" he stammered.
"So did I," Peter said roughly. "Good God, it can't be… This is getting a bit too weird to be pleasant. Look here… Damn it, that was no ghost. There must be a secret way through the wall." His torch played over the wall. It was built of great stone slabs each about four foot square. He began to feel them in turn. "We must be under the terrace," he said. "Gosh, don't you see? We're standing on the level of the ground here!" One of the blocks gave slightly under the thrust of his hand. "Got it!" he panted, and set his shoulder to it. It swung slowly outward, turning on some hidden pivot, and as it moved that hideous wail once more rent the stillness.
"So that's it, is it?" Charles said grimly. "Well, I don't mind telling you that I'm damned glad we've solved the origin of that ghastly noise." He squeezed through the opening in Peter's wake, and found himself, as Peter had prophesied, in the garden directly beneath the terrace. There was no sign of anyone amongst the shrubs near at hand, and it was obviously useless to search the grounds. After a moment both men slipped back into the cellar, and pushed the stone into place again.
"Might as well have a look round to see what that chap was after," Peter said. "Why the banging? Is he looking for a hollow wall, do you suppose? Dash it, I rejected hidden treasure as altogether too far-fetched, but it begins to look remarkably like it!"
"Personally I don't think we shall find anything," Charles answered. "Still, we can try. What a maze the place is!"
Together they explored all the cellars, but Charles was right, and there was nothing to be seen. Deciding that their nocturnal visitor would hardly attempt another entrance now that his way of ingress had been discovered, they made their way up the stairs again.
As they crossed the hall towards the library door a glimmer of light shone on the landing above, and Margaret's voice called softly: "Peter."
"Hullo!" Peter responded.
"Thank goodness!" breathed his sister, and came cautiously down to join him. In the lamplight her face looked rather pale, and her eyes very big and scared. "That awful groan woke me," she said. "I heard it twice, and called to you, Peter. Then when you didn't answer I went into your room and saw the bed hadn't been slept in. I got the most horrible fright."
"Don't make a row. Come into the library," Peter commanded. "You didn't wake Celia, did you?"
"No, I guessed you and Charles had staged something. Did you hear the groan? What have you been doing?"
"We not only heard it, but on two occasions we caused it," Peter said, and proceeded to tell her briefly all that had happened.
She listened in wondering silence, but when he spoke of the part he believed Strange to be playing, she broke in with an emphatic and somewhat indignant headshake. "I'm sure he isn't a crook! And I'm perfectly certain he'd never make awful noises to frighten us, or put skeletons where we should find them. Besides, why should he?"
"I'm not prepared to answer that question without due warning," Charles said cautiously. "All I know about him at present is that he's a rather mysterious fellow who holds distinctly fishy conversations with a palpable old lag, and who - apparently - knows how to get round persons of your sex."
"That's all rot," Margaret said without hesitation. "There's nothing in the least mysterious about him, and I expect if you'd heard more of it you'd have found that the fishy conversation was quite innocent really. You know how you can say things that sound odd in themselves, and yet don't mean anything."
"I hotly resent this reflection upon my conversation," Charles said.
"You've got to remember too, Peg, that when we heard that groan before, we found Strange close up to the house, and on the same side as the secret entrance," Peter interposed. "I don't say that that proves anything, but it ought to be borne in mind. I certainly think that Mr. Michael Strange's proceedings want explaining."
"I think it's utterly absurd!" Margaret said. "Why, you might as well suspect Mr. Titmarsh!" Having delivered herself of which scornful utterance, she rose, and announced her intention of going back to bed.
To be on the safe side, Charles and Peter spent the following morning in sealing up the hidden entrance. An account of the night's happenings did much to reconcile Celia to her enforced stay at the Priory. Human beings, she said, she wasn't in the least afraid of.
"I only hope," said Mrs. Bosanquet pessimistically, "that we are not all murdered in our beds."
Both she and Celia were agreed that the latest development made the calling in of police aid imperative. The men were still loth to do this, but they had to admit that Celia had reason on her side.
"There's no longer any question of being laughed at," she argued. "Someone broke into this house last night, and it's for the police to take the matter in hand. It's all very well for you two to fancy yourselves in the role of amateur detectives, but I should feel a lot easier in my mind if some real detectives got going."
"How can you?" said Charles unctuously. "When you lost your diamond brooch, who found it?"
"I did," Celia replied. "Wedged between the bristles of my hair-brush. That was after you'd had the waste up in the bath, and two of the floor-boards in our room."
"That wasn't the time I meant," said Charles hastily.
Celia wrinkled her brow. "The only other time I lost it was at that hotel in Edinburgh, and then you stepped on it getting out of bed. If that's what you mean…'
"Well, wasn't that finding it?" demanded Charles. "Guided by a rare intuition, I rose from my couch, and straightway put my - er - foot on the thing."
"You did. But that wasn't quite how you phrased it at the time," said Celia. "If I remember rightly…'
"You needn't go on," Charles told her. "When it comes to recounting incidents in which I played a prominent part you never do remember rightly. To put it bluntly, for gross misrepresentation of fact you're hard to beat."
"Time!" called Peter. "Let's put it to the vote. Who is for calling in the police, or who is not? Margaret, you've got the casting vote. What do you say?"
She hesitated. "I think I rather agree with Celia. You both suspect Mr. Strange. Well, I'm sure you're wrong. Let the police take over before you go and make fools of yourselves." She added apologetically: "I don't mean to be rude about it, but…'
"I'm glad to know that," said Charles. "I mean, we might easily have misunderstood you. But what a field of conjecture this opens out! I shall always wonder what you'd have said if you had meant to be rude."