"What's that?" yelled Kingsley, dropping the sheet.

The thought of the Engineer came to them as calm as ever, as absolutely devoid of emotion as it bad always been.

"The Hellhounds," he said "The Hellhounds are attacking us."

As he spoke, Gary watched the sheet of paper flutter to the floor, a little fluttering sheet that held the key to the riddle of the universe scratched upon it in the black scrawlings of a soft-lead pencil.

The Engineer moved across the laboratory to a panel. His metallic fingers reached out, deftly punched at studs. A wall screen lighted up and on it they saw the bowl of sky above the city. Ships were shooting up and outward, great silver ships that had grim lines of power about them. Up from the roofs they arrowed out into space, squadron after squadron, following a grim trail to the shock of combat. Going out to meet the Hellhounds.

The Engineer made adjustments on the panel and they were looking deeper into space, far out into the darkness where the atmosphere had ended. A tiny speck of silver appeared and rapidly leaped toward them, dissolving into a cloud of ships. Thousands of them.

"The Hellhounds," said the Engineer.

Gary heard Herb suck in his breath, saw Kingsley's hamlike hands clenching and unclenching.

"Stronger than ever," said the Engineer. "Perhaps with new and more deadly weapons, perhaps more efficient screens. I am afraid, so very much afraid, that this means the end of us… and of the universe."

"How far away are they?" asked Tommy.

"Only a few thousand miles now," said the Engineer. "Our alarm system warns us when they are within ten thousand miles of the surface. That gives us time to get our fleet out into space to meet them."

"Is there anything we can do?" asked Gary.

"We are doing everything we can," said the Engineer.

"But I don't mean you," said Gary. "Is there anything the five of us can do? Any war service we can render you?"

"Not now," said the Engineer. "Perhaps later there will be something. But not now."

He adjusted the screen again and in it they watched the defending ships of the Engineers shooting spaceward, maneuvered into far-flung battle lines — like little dancing motes against the black of space.

In breathless attention they kept their eyes fixed on the screen, saw the gleaming points of light draw closer together, the invaders and the defenders. Then upon the screen they saw dancing flashes that were not reflections from the ships, but something else — knifing flashes that reached out, probed and stabbed and slashed, like a searchlight's beam cuts into the night. A tiny pinpoint of red light flashed momentarily and then went out. Another flamed, like lightning bugs of a summer night, except the flash was red and seemed filled with a terrible violence.

"Those flashes," breathed Caroline. "What are they?"

"Exploding ships," said the Engineer. "Screens break down and the energy drains out and then an atomic bomb or ray finds its way into them."

"Exploding ships," said Gary. "But whose?"

"How can I tell?" asked the Engineer. "It may be theirs or ours."

Even as he spoke a little ripple of red flashes ran across the screen.

CHAPTER Ten

HALF the city was in ruins, swept and raked by the stabbing rays that probed down from the upper reaches of the atmosphere, blasted by hydrogen and atomic bombs that shook the very bedrock of the planet and shattered great, sky-high towers of white masonry into drifting dust. Twisted wreckage fell into the city from the battle area, great cruisers reduced to grotesque metal heaps, bent and burned and battered out of all semblance to a ship, scorched and crushed and flattened by the energy unloosed in the height of battle.

"They have new weapons," said the Engineer. "New weapons and better screens. We can hold them off a little longer. How much longer I do not know."

In the laboratory, located in the base of one of the tallest of the skyscrapers in the great white city, the Engineers and the Earthlings had watched the battle for long hours. Had seen the first impact of the fleets, had watched the first dogfight out at the edge of atmosphere, had witnessed the Hellhounds slowly drive the defenders back until the invaders were within effective bombardment distance of the city itself.

"They have a screen stripper," said the Engineer, "that is far more effective than anything we have ever seen. It is taking too much of our ships" energy to hold up their screens under this new weapon."

In the telescopic screen a brilliant blue-white flash filled all the vision-plate as a bomb smashed into one of the few remaining towers. The tower erupted with a flash of blinding light and disappeared, with merely the ragged stump of masonry bearing mute testimony to its once sky-soaring height.

"Isn't there anyone who can help us?" asked Kingsley. "Surely there is someone to whom we might appeal."

"There is no one," said the Engineer. "We are alone. For thousands of light years there are no other great races to be found. For millions of years the Hellhounds and the Engineers have fought, and it has always been those two and just those two alone. Thus it is now. Before, we have driven them off. Many times have we destroyed them almost to the point of anihilation that we might hold their cosmic ambitions under proper check. Now it seems they will be the victors."

"No other race," said Gary, musing, "for thousands of light years."

He stared moodily at the screen, saw a piece of twisted wreckage that had at one time been a ship crash into the stump of broken tower and hang there, like a bloody, smoke-blackened offering tossed on the altar of war.

"But there is," he said. "There is at least one great race very near to us."

"There is?" asked Caroline. "Where?"

"On the other universe," said Gary. "A race that is fully as great, as capable as the Engineers. A race that should be glad to help us in this fight."

"Great suffering snakes," yelped Herb, "why didn't we think of that before?"

"I do not understand," said the Engineer. "I agree they are a great race and very close to us. Much too close, in fact. But they might as well be a billion light years away. They can do us no good. How would you get them here?"

"Yes," rumbled Kingsley, "how would you get them here?"

Gary turned to the Engineer. "You have talked to them," he said. "Have you any idea of what kind of people they might be?"

"A great people," said the Engineer. "Greater than we in certain sciences. They are the ones who notified us of the danger of the approaching universes. They knew they were nearing our universe when we didn't even know there was another universe other than our own. Such very clever people."

"Talk to them again," said Gary. "Give them the information that will enable them to make a miniature universe… one of Caroline's hyperspheres."

"But," said the Engineer, "that would do no good."

"It would," said Gary, grimly, "if they could use the laws of space to form a blister on the surface of their universe. If they could go out to the very edge of their space-time frame and create a little bubble of space — a bubble that would pinch off, independent of the parent universe and exist independently in the five-dimension inter-space."

Gary heard the rasp of Kingsley's breath in his helmet phones.

"They could cross to our universe," rumbled the scientist. "They could navigate through the inter-space with complete immunity."

Gary nodded inside his helmet. "Exactly," he said.

"Why, Gary," whispered Caroline, "what a thought!"

"Boy," said Herb, "I can hardly wait to see them Hellhounds when we sic those fellows on them."

"Maybe," said Tommy, "they won't come."

"I will talk to them," said the Engineer.