Roger Kay drew a deep breath, his face suddenly serious. Three days to save the System from an invasion that could not possibly prove to be less than a major catastrophe, less than the end of things as he knew them. * * * * * Even now the invaders from Andromeda were approaching the System's outermost defenses; converging upon the virtually helpless garrisons on Pluto. Patrol spacers off the frigid planet had already contacted spearheads of the huge armada--with fatal results. Once before the System had been periled by these devils from the distant galaxy. Victory had been costly then, but the combined Planetary fleets could not now hope to stave off the full force on this new attack. They would have to yield space; fall back to more favorable positions. Trionite alone would prove the decisive factor in any war of worlds, and the United Planets had not been able to learn the secret of manufacturing the new explosive, one ton of which could wreck an invading army. As Roger Kay set the robot-course dial of his speedy helio for the mining settlement, he switched in for a moment on Wargan's private wave-band. "Leaving now, sir," he reported crisply. "Be there in two hours. Any further instructions?" "Do your best, Kay, that's all," came the weary voice of the S.B.I. chief. "New reports in confirm the old ones. We expect the first blow by noon Friday. Pluto is doomed; now being evacuated." "We've got to stop them," Roger Kay said fiercely as he snapped the switch. "We've just got to!" He settled back to get in a much-needed two hours of sleep while the robot pilot held his course. The alarm bell awakened him, and he pointed the craft down under the great red disk of Big Jupe, toward the low range of purple cliffs indicated on his map. A few minutes later he was knocking at the door of the dome-shaped laboratory. Ann North was twice as beautiful in the flesh as she had seemed on the visi-screen. Attired in the modish shorts and tunic that had become universal garb for Earth-women, she looked like a figure from a Grecian frieze. She led him to the library. "Dad's asleep at last," she said. "I persuaded him to rest for a few hours--on the strength of my argument that he'd accomplish more in the long run if he kept his brain clear." Roger Kay nodded understandingly. "I just had a bit of sleep myself en route. Nobody at headquarters has slept much the last few days. By the way, I'm woefully in the dark about a lot of things. Will you tell me just what your father's trying to re-discover? If you can enlighten me, I'll not have to ask him so many darn-fool questions." "You know, of course," said Ann North when they were comfortably seated, "that it's a ray that will explode any explosive at a distance. Or perhaps I shouldn't have said a ray--it's really a sound wave, in the ultra-sonic belt, traveling on a beam. It disrupts any unstable chemical compound."