He waited until the child had disappeared into the shop, then opened the door of Number 407 with the key which she had given him, walked in and closed the door behind him. Here he paused for a moment, thinking rapidly. He felt that he was up against a brain as agile as his own, and that the coming interview would demand all his skill if he were to bring it to a favourable issue. This man Lacey had taken steps to advertise the fact that Mr. Martin had entered the premises, obviously as a precaution against foul play. His automatic would be of no use to him, he would have to rely upon his brains instead. Then suddenly it flashed upon him that he had been given a very fortunate opportunity. It was not to be supposed that Lacey had left his discovery, whatever it was, lying about the cellar in full view of the workmen who had been there that morning. But, if he could have a quiet look round by himself, he might gain some clue to the nature of the discovery, and so be prepared to meet Lacey when he arrived. Mr. Martin was not anxious for Lacey to come upon him unawares while he was searching the cellar. Fortunately, the possibility of such an event could easily be guarded against. He bolted the door top and bottom, and noticed with satisfaction that the bolts were brand new, evidently part of the alterations which the new tenant was making to the premises. Then, after glancing round the ground floor, and making sure that nobody was hidden there, he made his way to the top of the stairway which led down to the cellars. The upper part of the house had no connection with the shop, and was approached by a separate entrance. He saw at once that the letter had been correct as to the alterations being made. During his tenancy the cellars had been lighted by gas, but now electric light was being installed throughout. Mr. Martin could see the run of the wires and the fittings for the lamps, but he noticed that the lamps themselves had not yet been inserted. This fact did not worry him. He had a box full of matches in his pocket, and one of these he lit as he cautiously descended the stairs. The builders’ litter was here, as everywhere, and he picked his way among it until he stood in the main vault below the shop. It smelt close and musty, as though it had only recently been opened up after a long period of disuse. Mr. Martin, holding a lighted match above his head, glanced rapidly round. Nothing had been altered here since he remembered it, except that a new brass switch stood behind the door, and two empty lamp sockets projected downwards from the ceiling.