He tried to grope for his keys, to unfasten the locked door of the cell and reach the open air. But the effort was lost in a confused, quaking gesture. He could not keep his hands steady for a second, as the violent spasm that heaved and tore at his breathing organs, fairly threw his whole body off balance! The keys jingled to the floor, and he tried to find them, feeling with his fingers. His streaming eyes were blinded, so that he could not see. Weakened and choked, he crumpled to his knees, and sprawled helplessly on his belly. But that smothering, drowning fit that wracked him, went on. From this point, the transition from humor to horror was swift. Bloody froth came to Arruj's lips. He writhed. His sneezes and coughs and raking gasps became less forceful with exhaustion, but more hideous, with the bubbling, scratching sound of an unmistakable death-rattle. All this, Ron Leiccsen watched, almost without moving. He was too fascinated, too puzzled, too unbelieving to move. But then, as if remembering a duty, he picked up Arruj's staff. It was quite massive. He lifted it, and aimed a blow at Arruj's skull. But the blow that would have pulped the Callistan overseer's gray matter, never was delivered. Ron felt suddenly sheepish--almost guilty. It was against best human principle to murder a helpless enemy. And Ron did not need the word of a physician to know that Arruj was dying. But how? Why? That was the question! Ron listened. Dimly, within the great, roaring factory, and beyond its walls he could hear more coughs and sneezes, like the rattle of great drums. No human chests of Earth could have produced such noises. Only the great barrel-like thoraxes of Callistans could ever reverberate like that! It was a plague, then. Something that must have stricken them all, suddenly. But how was it possible? They were tough, these beings from that moon of Jupiter. Earth-germs, for instance, did them no harm. And there were few native Acharian diseases that their rugged flesh could not throw off. Still, now, there was a pestilence among them--a killing horror, swift and strangling! Ron Leiccsen thought of Arne Reynaud, and wondered. Then he saw the keys there on the floor, beside Arruj's writhing, tortured form. He picked them up, chose the one he knew fitted the lock of his cubicle, and opened the door. Cautiously he stepped over the quivering, doomed Arruj. In the corridor outside, along the row of cells, other Callistans sprawled, helpless and strangled, their efforts to breathe consisting only of horrible, gurgling gasps. Something must be swiftly inflaming their lungs, until death by strangulation was inevitable. Like pneumonia or diphtheria, but far more rapid. In a daze of wonder, in which hope and optimism scarcely dared to rise, Ron rushed from one cubicle door to another. It was easy to release the human slaves who had worked the machines within each cell. All the doors could be unlocked with the same key as his own.