“This may account for the statement of Bolton that the door seemed more difficult to open last night than to-day,” he said.  “Unless I am greatly mistaken, some sort of attachment existed here until quite recently.”

“Possibly a contrivance for reclosing the door?” I suggested.

The marks in fact roughly corresponded to those which would be made by the presence of such a contrivance and there seemed to have been some attempt where it had been removed to disguise the holes left by the screws.

“But the purpose of it?” muttered Gatton helplessly.

“God knows,” I said; “the purpose of the whole thing is a mystery beyond me entirely.”

“Assuming that such a piece of mechanism as you suggest had been attached to the door,” mused Gatton, “you would have noticed its operation last night, unless one of you held the door open.”

“Neither of us held the door!” I interrupted excitedly.  “I remember that we stood just outside looking in.  I was behind the constable and he was directing the rays of his lantern into the place.”

“H’m,” muttered Gatton.  “Then it wasn’t a contrivance for closing the door; it was something else.  Suppose we investigate the other door?”

We proceeded to the other door and I became aware of an intense curiosity respecting what we should find, and of a conviction too that there would be evidence here of another attachment.  In this I was quite correct.  Some piece of mechanism had evidently been fastened to this door also.  Together we stood staring up at these tell-tale screw-holes and then rather blankly we stared at one another.

“We only lack one thing,” said Gatton; “the scheme upon which all these contrivances and apparently isolated episodes were hung together.  Nothing, as we have already assumed, was accident, and nothing coincidence.  It was with some deliberate purpose that the constable was instructed to walk through this garage, opening and shutting the doors behind him.”

“From whom did these instructions come?”

“That is one of the minor points which I have already cleared up,” he replied.  “On my way here I called at the house agent’s, as you know, since I have the keys; I also called at the station.  The sergeant who was on duty last night I could not see, unfortunately, but I learned—­that it was a woman who rang up.”

My heart sank lower and lower.  It seemed to me as we stood in that empty garage that an invisible hand was drawing a net closer and closer about Isobel and my ideas became increasingly chaotic, for the purpose of it all eluded me, try how I would to conceive of a scheme by which any one could profit which necessitated the imprisonment, or worse, of Isobel.

“And the agent?” I asked in a rather toneless voice.

Gatton shook his head.

“I have no reason to doubt the word of this man of business,” he replied, “because at the time when I saw him he could not possibly have learned of the crime, but nevertheless his account is almost unbelievable.  It appears then, he, too, received his instructions throughout by telephone.”