"Why should one enemy prevent another from killing us?" Emmett repeated wonderingly. He remembered another question that had nagged at his mind: _Why had the Agronians totally destroyed Earth?_ Why hadn't they eliminated Earthmen and preserved the planet for exploitation--as a colony, a military base, any one of a thousand uses? There was only one possible answer. A race might destroy a planet if it was useless. Earthmen had discovered useless planets, planets with poisonous atmospheres. Was Earth's atmosphere poisonous to the Agronians? One Agronian had prevented another from killing them with a viciousness and an urgency that indicated it had been a life-and-death necessity. Why? What would happen if they were to die? Something clicked in his mind and a startling certainty occurred to him. _Oxygen was poisonous to the Agronians!_ That was why his life had been spared. And the pilot's--and Gloria's. Their spacesuits would have been punctured and their oxygen supply would have spread with deadly rapidity throughout the room. Without hesitation he removed his helmet and adjusted the controls of his oxygenating machine until it was discharging oxygen at maximum capacity. With a shrill outcry the two aliens darted toward him. But a thin, ghostly vapor of oxygen spread rapidly through the fog-like atmosphere, and halted them in their tracks. "You deserve to die," Emmett whispered. The enemy collapsed at his feet and writhed helplessly on the floor. Their bodies quivered spasmodically and were still. * * * * * Gloria's hysterical, joyous laughter rang in his ears like triumphant bells, and through the Agronian atmosphere that burned his face and smarted his eyes he dimly saw George's image as he rushed to the control board. He held his breath but realized that his death was certain. He could never hold his breath long enough to replace the helmet and wait for the purifying agents to cleanse the poison that now filled his spacesuit. When he could hold his breath no longer, he inhaled quickly and deeply. It was like inhaling a warm, comforting darkness.... Transcriber's Note: This etext was produced from _Fantastic Universe_ November 1956. Extensive research did not uncover any evidence that the U.S. copyright on this publication was renewed. Minor spelling and typographical errors have been corrected without note.