Now I had been the last to visit the basement and had locked the door—­from force of habit, I suppose—­leaving the key in the lock.  It was still there.  And there is but one way to enter that basement:  through this one door, and no other.

“Who is it?” I called out peremptorily.  No answer; only a repetition of the pounds.

“What do you want?”—­louder.

“Open this door, quick!” cane a muffled reply.

The voice was unrecognisable.  I stood and thought quickly; then shouted: 

“Wait a minute, until I get a key!”

I motioned to Charlotte.  She tip-toed to my side.  I whispered something in her ear; and she slipped off into the kitchen, there to phone Miss Clarke and warn her to notify her colleagues at once.  And so, as I unlocked the door, I was fortified by the knowledge that I would be assisted by the combined mind-force of a score of highly developed intellects.

I was little surprised, a second later, to see that the intruder was Rhamda Avec.  What reason to expect anyone else?

“How did you get down there?” I demanded.  “Don’t you realise that you are liable to arrest for trespass?”

I said it merely to start conversation but it served only to bring a slight smile to the face of this professed friend of ours, for whom we felt nothing but distrust and fear.

“Let us not waste time in trivialities, Fenton,” he rejoined gently.  He brushed a fleck of cobweb from his coat.  “By this time you ought to know that you cannot deal with me in any ordinary fashion.”

I made no comment as, without asking my leave or awaiting an invitation, he stepped through into the dining-room and thence into the parlour.  I followed, half tempted to strike him down from behind, but restrained more by the fact that I must spare him than from any compunctions.  Seemingly he knew this as well as I, he was serenely at ease.

And thus he stood before Jerome and Ariadne.  The detective made a single exclamation, and furtively shifted his coat sleeves.  He was getting that infernal breast gun into action.  As for Ariadne, she stared at the new arrival as though astonished at first.

When Charlotte returned, a moment later, she showed only mild surprise.  She quietly took her chair and as quietly moved her hand so that the gem shone in full view of our visitor.

But he gave her and the stone only a single glance, and then rested his eyes upon our new friend.  To my anxiety, Ariadne was gazing fixedly at him now, her expression combining both agitation and a vague fear.

It could not have been due entirely to his unusual appearance; for there was no denying that this grey-haired yet young-faced man with the distinguished, courteous bearing, looked even younger that night than ever before.  No; the girl’s concern was deeper, more acute.  I felt an unaccountable alarm.

From Ariadne to me the Rhamda glanced, then back again; and a quick satisfied smile came to his mouth.  He gave an almost imperceptible nod.  And, keeping his gaze fixed upon her eyes, he remarked carelessly: