everything we possibly can already."

"I'm sure of that. But Mr. Kay is rather outstanding as a scientist himself, Miss North. He'll be able to help—at least in some of the detail work, to save time."

Roger Kay grinned. "He means, Miss North, that I can clean the test tubes and solder the wires and let your father save his energy for the brain-work."

His smile was infectious, and the scientist's daughter capitulated. Wargan flicked the switch and threw the screen into blankness.

"I'll give you an order for the fastest helio we have," he said. "You'll be there in three hours. And that means there will be a little less than three days left!"

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 

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