"It's all a question of alibi," he said. "We know perfectly well what an ingenious scheme Anstruther has put up so that he may be what an Irishman would call in two places at the same time. Here is a magnificent opportunity of getting to the bottom of that mysterious music business."

"Right you are," Rigby cried. "It would be like flying in the face of Providence to throw away such a chance. Anstruther is here, and likely to remain, and so is Serena. You may depend upon it that the other maid has gone to bed, so that we should have the house in Panton Square all to ourselves. You know the ropes better than I do, Jack. Can you tell us a good way of getting into the house without playing the burglar?"

Jack thought a moment, then an inspiration came to him again; the thing was quite simple.

"We can walk into the place as if it belonged to us," he said. "When Claire came away, Anstruther told her that he should retire early. Claire did not wish to keep the servants up unduly, so she took a latch-key with her."

"Absolutely made for us," Rigby exclaimed. "You go off to Miss Helmsley and borrow her latch-key, and we will get to the bottom of the whole mystery whilst Anstruther is enjoying himself here."

Jack came back presently with the latch-key in his possession. It was an easy matter to get out of the house without being observed; then a cab was called, and the two proceeded to Jack's chambers, where they stripped off their fancy dresses hastily and assumed more civilized attire.

"I vote we take Bates into this business," Rigby suggested. "I've got a little idea of my own, which I will tell you about after we have been to Panton Square."

Unfortunately the services of Inspector Bates were not available, for he had been called out on some business of importance, and was not expected back till the following morning.

"We shall have to go through it ourselves," Jack said. "You will have a fine lot of copy for the Planet a bit later on. I declare I am getting quite fascinated by my present occupation. Shall we take a cab, or would it not be more safe for us to walk?"

Panton Square was reached at length, and No. 5 appeared to be in total darkness. As the friends had anticipated, Serena's fellow servant had gone to bed, for neither at the front or back of the house was there so much as a glimmer of light to be seen. An application of the latch-key to the door proved quite successful, and a minute later the two friends were inside. They had not the slightest hesitation in putting up the lights, so that the passing police might infer that the occupants of the place had returned. Not that he wanted to trouble much about anything but the study, seeing that it was there that the mysterious music always emanated.