LIANG-SUN chanced to turn around at this moment. He saw Doc, saw what had happened. His piping yell of fright sounded like a rat squeal in the dingy Chinatown street. He fled.

A bronze blur of speed, Doc raced after him.

Liang-Sun was fumbling inside the waistband of his trousers. He brought out his sword. Evidently he carried it in a sheath strapped next to his leg.

Doc overhauled him rapidly. The Mongol was only a hundred feet away — seventy-five — fifty.

Then a big policeman, attracted by Liang-Sun's yell of fear, popped around a corner. He stood directly in Liang-Sun's path, revolver in hand.

The Mongol was desperate. He slashed his sword at the cop and the cop shot him, killing him instantly.

The policeman had acted instinctively in defense of his life. He watched Doc come up.

"Sure, an' this is the first man I ever killed. I hope he needed it," the cop spoke.

He eyed Doc suspiciously. He did not know the bronze giant.

"Was ye chasin' this bird?" he demanded.

"I was," Doc admitted. "And don't let the fact that you killed him bother you. He is a murderer, probably several times over. He killed a man at the home of Scott S. Osborn to-night. And I think he must have committed other crimes at the residence of Scott S. Osborn's brother not very many minutes ago."

Doc did not know what had happened in the dwelling of Scott S.. Osborn's brother. But the fact that Ham had chased Liang-Sun out showed something had gone wrong.

The cop was suspicious of Doc.

"Yez jest stick around here, me b'y!" he directed. "We'll want to ask yez a lot av questions."

Doc shrugged.

The officer slapped big hands over Doc's person in search of a gun. The fact that he did that was unfortunate. He broke one of the anaesthetic balls in Doc's pockets.

A minute afterward, he was stretched on his back on the walk, snoring loudly.

Doc left the cop where he lay. The fellow would revive after a time, none the worse for his slumber.

From a near-by call box, Doc turned in an alarm to the police station. He did not give his name.

He hurried back to get the two peddlers who had been following him. They should be asleep on the walk.

But they weren't! Some denizen of Chinatown had moved them. Doc knew it must have been the work of some of the Mongol horde.

Chinatown, despite all the fiction written about it, was actually one of the quietest sections in the city. No legitimate resident of the district would court trouble by assisting the unconscious pair.

A brief, but intensive search disclosed no sign of the vanished two.

HALF an hour later, Doc was in his skyscraper office uptown. None of his five men had returned.

With a chemical concoction from the laboratory, Doc erased the invisible writing off the window. Then he inscribed a fresh message there.

Swinging out into the corridor, he rode the button until an elevator came up. The cage doors opened noiselessly, let him in, and closed. There was a windy sigh of a sound as the lift sank.

Adjoining Doc's office was a suite which had been empty some months. Rents were high up here in the clouds, and times were tough, so many of the more costly offices were without tenants.

It would have taken a close examination to show the door of this adjoining suite had been forced open.

Inside, a man was just straightening from a large hole which had been painstakingly cut in the wall of Doc's office. The actual aperture into Doc's sanctum was no larger than a pin head. But by pressing an eye close, an excellent view could be obtained.

The watcher was a round-faced, lemon-skinned Oriental. He hurried out and tried to force the door of Doc's office. The lock defied him. The door was of heavy steel — Doc had put that steel in to discourage Renny's joyful habit of knocking the panels out with his huge fists.

Returning to the vacant suite, the Oriental set to work enlarging his peep-hole. He used an ordinary pick. In ten minutes, he had opened an aperture in the plaster and building tile which would admit his squat frame.

He crawled in. First, he made sure the corridor door could be locked from the inside. He left it slightly ajar.

The window next received his attention. He had watched Doc write upon it. Yet he could discern no trace of an inscription.

Working with great care, the Mongol removed the pane of glass. He carried it outside. He was going to take it to a place where some one with an understanding of invisible inks could examine it. He rang for the elevator.

The elevator operator eyed him doubtfully as they rode down.

"You work here?" he demanded.

"Wolk this place allee time now," singsonged the Mongol. He grinned, wiped his forehead. "Allee same wolk velly much and get velly little money.

The operator was satisfied. He hadn't seen this man before. But who would go to the trouble of stealing a sheet of plate glass?

The cage stopped at the ground floor. The Mongol bent over to pick up his glass.

What felt like a steel trap suddenly got his neck.

THE slant-eyed man struggled desperately. The hands on his throat looked bigger than gallon buckets.

They were Renny's hands — paws that could knock the panel out of the heaviest wooden door.

Monk, Long Tom and Johnny danced about excitedly outside the elevator. They all had just come in.

"Hey!" Monk barked. "How d'you know he's one of the gang?"

They were nearly as surprised as the Mongol at Renny's sudden act.

"He's got the window out of Doc's office, you homely goat!" Ham snapped, after a glance into the elevator.

"Yeah!" Monk bristled. "How can you tell one hunk of glass from another?"

"That is bullet-proof glass," Ham retorted. "So far as I know, Doc has the only office in this building with bulletproof windows."

Monk subsided. Ham was right.

Renny and the Oriental were still fighting. The Oriental launched frenzied blows, but he might as well have battered a bull elephant, for all the effect they had.

Desperate, the Mongol clawed a knife out of a hidden sheath.

"Look out, Renny!" Monk roared.

But Renny had seen the knife menace. He hurled the slant-eyed man away. The fellow spun across the tiled floor. He kept a grip on his blade.

Bounding to his feet, he drew back his arm to throw the knife.

Wham! A gun had appeared magically in Long Tom's pale hand, and loosed a clap of a report.

The bullet caught the Mongol between the eye — and knocked him over backward. His knife flew upward, pointfirst, and embedded in the ceiling.

A cop, drawn by the shot, ran in, tweeting excitedly on his whistle.

There was no trouble over the killing, though. Long Tom, as well as Monk, Renny, Ham and Johnny, held high honorary commissions in the New York police force.

Within a quarter of an hour, the five were up in the eighty-sixth floor office examining the pane of glass with ultra-violet light.

The message Doc had written upon it, flickered in weird bluish curves and lines. They read:

To RENNY: The chief of this Mongol gang

sometimes uses an office Iisted under

the name of the Dragon Oriental Goods Co.

It is the center, front office on the tenth

floor of the Far East building on lower

Broadway. A new skyscraper is going

up across the street.

Your engineering training will enable

you to get a structural steelworker's job

on the new building, Renny. Watch the office

of the Dragon Oriental Goods Co., and trail

any one you see using it.

With the chemical eraser, Monk carefully cleaned the glass plate. They were taking no chances on the leader of the Mongols getting hold of it. Such a misfortune might mean Renny's finish.