Suddenly my supersensitive mind was aware of a swift tension in the air. The leper hobbled out the door, closed it behind him. My eyes instinctively sought the couch whereon lay the man who had aroused my suspicions earlier in the day. I could have sworn that cold steely eyes gleamed menacingly before they flickered shut. I crossed to the couch in one stride and bent over the prostrate man. Something about his face seemed unnatural--a healthy bronze seemed to underlie the pallor of complexion. "Yun Shatu!" I shouted. "A spy is in the house!" Things happened then with bewildering speed. The man on the couch with one tigerish movement leaped erect and a revolver gleamed in his hand. One sinewy arm flung me aside as I sought to grapple with him and a sharp decisive voice sounded over the babble which broke forth: "You there! Halt! Halt!" The pistol in the stranger's hand was leveled at the leper, who was making for the door in long strides! All about was confusion; Yun Shatu was shrieking volubly in Chinese and the four China boys and Yussef Ali were rushing in from all sides, knives glittering in their hands. All this I saw with unnatural clearness even as I marked the stranger's face. As the flying leper gave no evidence of halting, I saw the eyes harden to steely points of determination, sighting along the pistol barrel--the features set with the grim purpose of the slayer. The leper was almost to the outer door, but death would strike him down ere he could reach it. And then, just as the finger of the stranger tightened on the trigger, I hurled myself forward and my right fist crashed against his chin. He went down as though struck by a trip-hammer, the revolver exploding harmlessly in the air. In that instant, with the blinding flare of light that sometimes comes to one, I knew that the leper was none other than the Man Behind the Screen! I bent over the fallen man, who though not entirely senseless had been rendered temporarily helpless by that terrific blow. He was struggling dazedly to rise but I shoved him roughly down again and seizing the false beard he wore, tore it away. A lean bronzed face was revealed, the strong lines of which not even the artificial dirt and grease-paint could alter. Yussef Ali leaned above him now, dagger in hand, eyes slits of murder. The brown sinewy hand went up--I caught the wrist. "Not so fast, you black devil! What are you about to do?" "This is John Gordon," he hissed, "the Master's greatest foe! He must die, curse you!" John Gordon! The name was familiar somehow, and yet I did not seem to connect it with the London police nor account for the man's presence in Yun Shatu's dope-joint. However, on one point I was determined. "You don't kill him, at any rate. Up with you!" This last to Gordon, who with my aid staggered up, still very dizzy. "That punch would have dropped a bull," I said in wonderment; "I didn't know I had it in me."