The Jew silently and promptly set out in the wake of the hurrying woman; presently she and her pursuer disappeared round a corner.  
"That's the result of our call, Mapperley!" said Hetherwick. "She's gone somewhere—to tell somebody!"
 
"Likely!" assented Mapperley. "But wherever she's gone, Issy Goldmark'll spot her. He's the eyes of a lynx."
 
"He let Baseverie slip him, the other night, though," remarked Hetherwick.
 
"Well, there was some excuse for that," said Mapperley, "to begin with, he was only instructed to find out where Baseverie went, and to end with he had found out! He'll not let this woman slip him. She's good to follow—plenty of her."
 
"I wish we knew what she'd left in that house," said Hetherwick. "We'll have to find out, somehow!"
 
"That's a police job," replied Mapperley. "Can't walk into people's houses without a warrant. And you say Matherfield's on the other track? However, I should say that this woman's gone off now to find somebody who's principally concerned—she looked afraid, in my opinion, when she saw me."
 
"She's in it, somehow," muttered Hetherwick.
 
"That house looks mysterious enough for anything. We'll keep a close watch on it, anyway, until Goldmark comes back, however long that may be."
 
But the Jew was back within twenty minutes. So was the woman. She came first, hurrying up the street quicker than when she had left it. As far as the watchers could make out from their vantage point, twenty yards away from her door, she looked flustered, distressed, upset. After her, on the opposite pavement, came Mr. Issy Goldmark, his hands in his pockets.
 
The woman re-entered the house; they heard the door bang. A moment later the Jew turned into the entry in which Hetherwick and Mapperley stood, half hidden from the street. He smiled, inscrutably.
 
"Thee her go back to her houth?" he asked. "Well, I followed. I thaw where thee'th been, too."
 
"Where, then?" demanded Hetherwick, impatiently.
 
Goldmark jerked his head in the direction from whence he had come.
 
"Round that corner," he said, "you get into a regular thlum. Little thtreeth, alleyth, pathageth, and tho on. In one of 'em, a narrow plathe, where there'th a thort of open-air market, there'th a good thithed pieth of blank wall, with an iron-fathen'd door in it. Well, the woman went in there—let herthelf in with a key that thee took from her pocket. Ath thoon ath thee'd gone in, I took a clother look. The door'th fathen'd with iron, or thteel, ath I thaid—jolly thtrong. There ain't no name on it, and no keyhole that you can look through. The wall'th a good nine or ten feet high, and it'th covered with broken glath at the top. Not a nithe plathe to get into, nohow!"
 
"Well?" inquired Hetherwick. "She went in?"
 
"Went in, ath I thay, mithter, and the door clothed on her. After I'd taken a glimpth at the door I got a potht behind one of the thtalls in the thtreet and watched. She came out again in about ten minitth—looked to me, too, ath if thee hadn't had a very plethant time inthide. Upthet! And thee thet off back here, fathter than vhat thee came. Now thee'th gone into her houth again—ath you no doubt thaw. And that'th all. But if I wath you, mithter," concluded Issy, "I should jutht find out vhat there ith behind that door and the wall it'th thet in—I thhould tho!"
 
"That's a police job," said Mapperley once more. "If we'd only got Matherfield with us, we could——" Hetherwick paused—thinking. "Look here, Mapperley," he continued, with a sudden inspiration. "I know what we'll do! You get a taxi-cab, as quickly as possible. Drive to the police station where I usually meet Matherfield. There's another man there whom I know, and who's pretty well up in this business—Detective-Sergeant Robmore. Ask for him. Tell him what we've discovered, and ask him to come back with you and to bring another man if he thinks it necessary. Now then, Goldmark! Tell Mapperley exactly where this place is."
 
The Jew pointed along the street to its first corner.
 
"Round that corner," he said. "Firtht turning to the right; then firtht to the left; then firtht to the right—that'th the thpot. Lot'th o' little thtallth in it—a bithy, crowded plathe."
 
"Didn't ye notice the name?" demanded Mapperley, half scoldingly.
 
"To be thure I did!" grinned Goldmark. "Pencove Thtreet. But it'th better to dethcribe it than to name it. And don't you go tellin' no tackthy-driver to drive you in there!—cauth' there ain't room!"
 
Mapperley gave no answer to this piece of advice; he shot off in the direction of Victoria Street, and Hetherwick turned to the Jew.
 
"We'll go and have another look at this place, Goldmark," he said. "But we'll go separately—as long as we're in this street, anyway. You stroll off to that first corner, and I'll join you."
 
He crossed the street when the Jew had lounged away, and once more took a narrow look at the house into which the big woman had vanished. It was as close barred and curtained as ever; a veritable place of mystery. For a moment Hetherwick doubted whether he ought to leave it unwatched. But the descriptions of the wall and door in Pencove Street had excited his imagination, and he went on, turned the corner, and rejoined Goldmark. Goldmark at once went in front, piloting him into a maze of unusually dirty and crowded streets, and finally into one, narrower than the rest, on each side of which were tent-like stalls whereon all manner of cheap wares were being offered for sale by raucous-voiced vendors. He saw at once that this was one of those open-air markets of which there are many in the poorer neighbourhoods of London, and wherein you can buy a sixpenny frying-pan as readily as a paper of fried fish, and a gay neckerchief alongside a damaged orange.
 
Threading his way behind Issy, and between the thronged stalls and the miserable shops that lined the pavement, Hetherwick presently came to the piece of blank wall of which the Jew had told him. The houses and shops round about were old and dilapidated, but the wall was either modern or had been rebuilt and strengthened. It stretched between two low houses, one used as a grocer's, the other as a hardware shop. In length, it was some thirty feet; in height, quite ten; its coping, as Goldmark had said, was liberally embattled with broken glass. The door, set flush with the adjoining masonry, was a solid affair, faced with metal, newly painted, and the lock was evidently a patent one. A significant fact struck Hetherwick at once—there was no sign of a bell and none of a knocker.
 
"You say the woman let herself in here?" he asked, as he and Issy paused.
 
"That'th it, mithter Hetherwick—let herthelf in," replied Issy. "I thee her take the key from her pocket."
 
Hetherwick glanced at the top of the wall.
 
"I wonder what's behind?" he muttered. "Building of some sort, of course." He turned to a man whose stall stood just in front of the mysterious door, and who at that moment had no trade. "Do you know anything about this place?" he asked. "Do you know what's behind this wall? What building it is?"
 
The stall-keeper eyed Hetherwick over, silently and carefully. Deciding that he was an innocent person and not a policeman in plain clothes, he found his tongue.
 
"I don't, guv'nor!" he answered. "'Aint a bloomin' notion! I been comin' here, or hereabouts, this three year or more, but I 'aint never seen behind that wall, nor in at that there doorway. S'elp me!"
 
"But I suppose you've seen people go in and come out of the door?" suggested Hetherwick. "It must be used for something!"
 
"I reckon it is, guv'nor, but I don't call nobody to mind, though, to be sure, I see a woman come out of it a while ago—big, heavy-jawed woman, she was. But queer as it may seem, I don't call to mind ever seeing an............