The housekeeper entered. "Now, Hutt," said Martin Hewitt, "you were saying yesterday, I think, that the main front door was the only entrance and exit for this building?" "That's so, sir--the only one as anybody can use, except me." "Oh! then there _is_ another, then?" "Well, not exactly to say an entrance, sir. There's a small private door at the back into the court behind, but that's only opened to take in coals and such, and I always have the key. This house isn't like yours, sir; you have no back way into the court as we have. It's a convenience, sometimes." "Ah, I've no doubt. Do you happen to have the key with you?" "It's on the bunch hanging up in my box, sir. Shall I fetch it?" "I should like to see it, if you will." The housekeeper disappeared, and presently returned with a large bunch of keys. "This is the one, Mr. Hewitt," he explained, lifting it from among the rest. Hewitt examined it closely, and then placed beside it one from the bunch Plummer had given him. "It seems you're not the only person who ever had a key exactly like that, Hutt," he said. "See here--this was found in Mr. Denson's pocket." Plummer nodded sagaciously. "All in the plant," he said. "See--it's brand new; clean as a new pin, and file marks still on it." "Take us to this back door, Hutt," Hewitt pursued. "We'll try this key. Is there a back staircase?" There _was_ a small back staircase, leading to the coal-cellars, and only used by servants. Down this we all went, and on a lower landing we stopped before a small door. Hewitt slipped the key in the lock and turned it. The door opened easily, and there before us was the little courtyard which I think I have mentioned in one of my other narratives--the courtyard with a narrow passage leading into the next street. Martin Hewitt seemed singularly excited. "See there," he said, "that is how Denson left the building without passing the housekeeper's box! And now I'm going to make another shot. See here. This key on Denson's bunch attracted my attention because of its noticeable newness compared with most of the others. _Most_ of the others, I say, because there is one other just as bright--see! This small one. Now, Hutt, do you happen to have a key like that also?" Hutt turned the key over in his hand and glanced from it to his own bunch. "Why, yes, sir!" he said presently. "Yes, sir! It's the same as the key of the fire-hose cupboards!" "Does that key fit them all? How many fire-hose cupboards are there?" "Two on each floor, sir, one at each end, just against the mains. And one key fits the lot." "Show us the nearest to this door." A short, narrow passage led to the main ground-floor corridor, where a cupboard lettered "Fire Hose" stood next the main and its fittings. "We have to keep the hose-cupboards locked," the housekeeper explained apologetically, "'cause o' mischievous boys in the offices."