Jack looked inquiringly at his friend. He had not yet fully grasped the significance of Rigby's remark. He asked for an explanation. Rigby went on to speak rapidly.

"It's like this, you see," he remarked. "When I saw that fellow just now and got him to play to me, a rather good idea came into my mind. So long as Anstruther can manage to delude us into believing that he spends most of his evenings in playing classical music, we can't get much further. Classical music is open to everybody; and if we allege that on a certain evening Anstruther performed one of Beethoven's sonatas--or, rather, that Padini performed it--we should have great difficulty in proving our point."

"I think I can catch your idea," Jack said.

"I thought you would. My idea was to get something original; something, if possible, that Anstruther has never even heard. He couldn't very well play a piece he had never heard, now could he? I asked Padini if he had anything of the kind in hand, and he played the piece which you so much liked. As I said just now, I have the thing in my pocket; and by means of that simple sheet of paper we are going to trap Anstruther."

"I don't quite see it," Jack said.

"What I mean is that we are going to manage it between us. Unless I am greatly mistaken, events will move very rapidly to-morrow night. Anstruther must of necessity be out most of the time after dinner, but the music in the study will go on all the same. You must manage to dine in Panton Square to-morrow night, and I will work the thing from the Great Metropolitan Hotel with Padini. In the course of the evening Padini will play the melody which we are now talking about, and you will hear it. Now, I know Miss Helmsley is a very capable pianist, and I want her to follow the air carefully, so that she will be able to play it by ear. Then we shall be in a position to ask Anstruther the name of the piece that attracted her so much. Miss Helmsley can pick it out on the piano for him, and ask him to play it again. You can imagine his difficulty, but you can hardly imagine a way out of it. This is only a side issue, I know; but it will all tell when we bring Anstruther to book and expose the whole conspiracy."

Jack appreciated the point, and promised to do his best to bring the comedy to a successful issue. There was nothing for it now but to reassume their fancy dresses and return to Belgrave Square.

By this time a considerable number of the guests were moving on elsewhere, though the majority of those present meant to see the thing through. As the cab bearing Jack and Rigby drove up they saw the tall figure of Anstruther coming down the steps. He stood there as if hesitating for a moment, then called a passing cab and gave some directions to Piccadilly.

"Any money I know where he is going to," Rigby said. "My dear fellow, you go inside and see Miss Helmsley, whilst I take this cab back to our rooms and change again into civilized attire."

"What are you going to do now?" Jack asked.

"I am going to follow Anstruther," Rigby explained. "I feel so restless to-night that I can't settle down to anything. So I am just going to follow that fellow, who is most assuredly going to see Carrington."

It was half-an-hour later before Rigby found himself, minus his fancy dress, in Piccadilly opposite the rooms occupied by Carrington. It was very late now, and Piccadilly was absolutely deserted, save for a passing policeman and a stray night cab whose driver appeared to be asleep upon the box. Rigby hesitated for a moment, a little uncertain as to what to do.

There was no difficulty in ascertaining as to whether Carrington had or had not gone to bed, for the lights were up in his sitting-room, and presently a shadow appeared upon the blind. Doubtless this was Carrington, and all speculation was set at rest an instant later by a second shadow on one of the blinds. The gigantic headdress of Anstruther loomed large against the light. There was nothing for it now but to wait patiently upon the course of events. Rigby pulled at the leg of the slumbering cabman, and brought him to a sense of his responsibilities.

"I don't want to take your cab anywhere," he explained. "All I want is to hire it for an hour or so and sit inside. You can go to sleep again if you like, and I'll wake you when I am ready to go. It will be an easy way of earning half-a-sovereign."

The cabman grinned and nodded as Rigby disappeared into the recesses of the cab. It was, perhaps, an hour later before the door leading to Carrington's flat opened and Anstruther came out. Evidently he had left his fancy dress behind him, for he was attired in a rough coat and deerstalker hat. Carrington appeared to be dissuading his friend from something, and Rigby could hear the latter laugh in reply.

"I tell you it must be done," Anstruther said, "and it will have to be done to-morrow night. I shall see friend Charlie without delay. If he is not in, I shall leave a settled note for him."

Anstruther strode off down the street, and presently hailed another night cab which was crawling down the road. Rigby sat up and aroused his own driver.

"Here's another five shillings for you," he said. "Keep that cab in front of you in sight, and follow it till it stops. Then you shall have fifteen shillings. Unless I am greatly mistaken, you will not have very far to go."

As a matter of fact, Rigby had summed up the situation quite correctly. The mention of the name of Charlie had given him the clue he required, this same Charlie being none other than the professional cracksman who had been engaged by Anstruther to deliver the letter at the Great Metropolitan Hotel to Ferris. This deduction proved to be absolutely correct, for a little time later the first cab pulled up in front of the tenement house where Seymour had taken up his temporary quarters. Rigby dismissed the cab, and followed cautiously. He was in time to see Anstruther take a key from his pocket, and let himself quietly into the rooms occupied by the individual who was known to his fr............