"Morey, pull down the wall over that door to block their passage," Arcot ordered. "I'll get the other wall."

Arcot pointed his pistol and triggered it. The outer wall flew outward in an explosion of flying masonry. He switched on his radio and called the Ancient Mariner.

"Wade! We were cut off because of the metal in the walls! We've been doublecrossed—they tried to jump us. Torlos warned us in time. We've torn out the wall; just hang outside with the airlock open and wait for us. Don't use the rays, because we'll be invisible, and you might hit us."

Suddenly the room rocked under an explosion, and the debris Morey's ray had torn down over the door was blasted away. A score of men leaped through the gap before the dust had settled. Morey beamed them down mercilessly before they could fire their weapons.

"In the air, quick!" Arcot yelled. He turned on his power suit and rose into the air, signalling Torlos to grab his ankles as he had done before. Morey slammed another parting shot toward the doorway as he lifted himself toward the ceiling. Then both Earthmen snapped on their invisibility units. Torlos, because of his direct contact with Arcot, also vanished from sight.

More of the courageous, but foolhardy Satorians leaped through the opening and stared in bewilderment as they saw no one moving. Arcot, Morey, and Torlos were hanging invisible in the air above them.

Just then, the shining bulk of the Ancient Mariner drifted into view. They drew back behind the wall and sought shelter. One of them began to fire his compressed air gun at it with absolutely no effect; the heavy lux walls might as well have been hit by a mosquito.

As the airlock swung open, Arcot and Morey headed out through the breach in the wall. A moment later, they were inside the ship. The heavy door hissed closed behind them as they settled to the floor.

"I'll take the controls," Arcot said. "Morey, head for the rear; you take the moleculars and take Torlos with you to handle the heat beam." He turned and ran toward the control room, where Wade and Fuller were waiting. "Wade, take the forward molecular beams; Fuller, you handle the heat projector."

Arcot strapped himself into the control chair.

Suddenly, there was a terrific explosion, and the titanic mass of the ship was rocked by the detonation of a bomb one of the men in the building had fired at the ship.

Torlos had evidently understood the operation of the heat beam projector quickly; the stabbing beam reached out, and the great tower, from floor to roof, suddenly leaned over and slumped as the entire side of the building was converted into a mass of glowing stone and molten steel. Then it crashed heavily to the ground a half mile below.

But already there were forty of the great battleships rising to meet them.

"I think we'd better get moving," Arcot said. "We can't let a magnetic ray touch us now; it would kill Torlos. I'm going to cut in the invisibility units, so don't use the heat beams whatever you do!"

Arcot snapped the ship into invisibility and darted to one side. The enemy ships suddenly halted in their wild rush and looked around in amazement for their opponent.

Arcot was heading for the magnetic force field which surrounded the city when Torlos made a mistake. He turned the powerful heat beam downwards and picked off an enemy battleship. It fell, a blazing wreck, but the ray touched a building behind it, and the ionized air established a conducting path between the ship and the planet.

The apparatus was not designed to make a planet invisible, but it made a noble effort. As a result one of the tubes blew, and the Ancient Mariner was visible again. Arcot had no time to replace the tube; the Satorian fleet kept him too busy.

Arcot drove the ship, shooting, twisting upward; Wade and Morey kept firing the molecular beams with precision. The pale rays reached out to touch the battleship, and wherever they touched, the ships went down in wreckage, falling to the city below. In spite of the odds against it, the Ancient Mariner was giving a good account of itself.

And always, Arcot was working the ship toward the magnetic wall and the base of the city.

Suddenly, giant pneumatic guns from below joined in the battle, hurling huge explosive shells toward the Earth-ship. They managed to hit the Ancient Mariner twice, and each time the ship was staggered by the force of the blast, but the foot-thick armor of lux metal ignored the explosions.

The magnetic rays touched them a few times, and each time Torlos was thrown violently to the floor, but the ship was in the path of the beams for so short a time that he was not badly injured. He more than made up for his injuries with the ray he used, and Morey was no mean gunner, either, judging from the work he was doing.

Three ships attempted to commit suicide in their efforts to destroy the Earthmen. They were only semi-successful; they managed to commit suicide. In trying to crash into the ship, they were simply caught by Morey's or Wade's molecular beam and thrown away. Morey actually developed a use for them. He caught them in the beam and used them as bullets to smash the other ships, throwing them about on the molecular ray until they were too cold to move.

Arcot finally managed to reach the magnetic wall.

"Wade!" he called. "Get that projector building!"

A molecular beam reached down, and the black metal dome sailed high into the sky, breaking the solidity of the magnetic wall. An instant later, the Ancient Mariner shot through the gap. In a few moments, they would be far away from the city.

Torlos seemed to realize this. Moving quickly, he pushed Morey away from the molecular beam projector, taking the controls away from him.

He did not realize the power of that ray; he did not know that these projectors could move whole suns out of their orbits. He only knew that they were destructive. They were several miles from the city when he turned the projector on it, after twisting the power control up.

To his amazement, he saw the entire city suddenly leap into the air and flash out into space, a howling meteor that vanished into the cloudbank overhead. Behind it was a deep hole in the planet's surface, a mighty chasm lined with dark granite.

Torlos stared at it in amazement and horror.

Arcot turned back slowly, and they sailed over the spot where the city had been. They saw a dozen or so battleships racing away from them to spread the news of the disaster; they were the few which had been fortunate enough to be outside the city when the beam struck.

Arcot maneuvered the ship directly over the mighty pit and sank slowly down, using the great searchlights to illuminate the dark chasm. Far, far down, he could see the solid rock of the bottom. The thing was miles deep.

Then Arcot lifted the ship and headed up through the cloud layer and into the bright light of the great yellow sun, above the sea of gray misty clouds.

Arcot signalled Morey, who had come into the control room, to take over the controls of the ship. "Head out into space, Morey. I want to find out why Torlos pulled that last stunt. Wade, will you put a new tube in the invisibility unit?"

"Sure," Wade replied. "By the way, what happened back there? We were surprised as the very devil to hear you yelling for help; everything seemed peaceful up to then."

Arcot flexed his bruised hands and grinned ruefully. "Plenty happened." He went on to explain to Wade and Fuller what had happened in their meeting with the Satorian Commander.

"Nice bunch of people to deal with," Wade said caustically. "They tried to get everything and lost it all. We would have given them plenty if they'd been decent about it. But what sort of war is this that the people of these two planets are carrying on, anyway?"

"That's the question I intend to settle," replied Arcot. "We haven't had an opportunity to talk to Torlos yet. He had just admitted to me that he was a spy for Nansal when the fun began, and we've been too busy to ask questions ever since. Come on, let's go into the library."

Arcot indicated to Torlos that he was to go with him. Wade and Fuller followed.

When they had all seated themselves, Arcot began the telepathic questioning. "Torlos, why did you force Morey to leave the ray and then destroy the city? You certainly had no reason to kill all the non-combatant women and children in that city, did you? And why, after I told you absolutely not to use the heat beam while we were invisible, did you use the rays on that battleship? You made our invisibility break down and destroyed a tube. Why did you do this?"

"I am sorry, man of Earth," replied Torlos. "I can only say that I did not fully understand the effect the rays would have. I did not know how long we would remain invisible; the thing has been accomplished in our laboratories, but only for fractions of a second, and I feared we might become visible soon. That was one of their latest battleships, equipped with a new, secret, and very deadly weapon. I do not know exactly what the weapon is, but I knew that ship could be deadly against us, and I wanted to make sure we were not attacked by it. That is why I used the beam while your ship was invisible.

"And I did not intend to destroy the city. I was only trying to tear up the factory that builds these battleships; I only wanted to destroy their machines. I had no conception of the power of that ray. I was as horrified to see the city disappear as you were; I only wanted to protect my people." Torlos smiled bitterly. "I have lived among these treacherous people for many years, and I cannot say that I had no provocation to destroy their city and everyone in it. But I had no intention of doing it, Earthman."

Arcot knew he was sincere. There could be no deception when communicating telepathically. He wished he had used it when communicating with the Commanding One of Sator; the trouble would have been stopped quickly!

"You still do not have any conception of the magnitude of the power of that beam, Torlos," Arcot told him. "With the rays of this ship, we tore a sun from its orbit and threw it into another. What you did to that city, we could do to the whole planet. Do not tamper with forces you do not understand, Torlos.

"There are forces on this ship that would make the energies of your greatest battleship seem weak and futile. We can race through space a billion times faster than the speed of light; we can tear apart and destroy the atoms of matter; we can rip apart the greatest of planets; we can turn the hurtling stars and send them where we want them; we can curve space as we please; we can put out the fires of a sun, if we wish.

"Torlos, respect the powers of this ship, and do not release its energies unknowingly; they are too great."

Torlos looked around him in awe. He had seen the engines—small, apparently futile things, compared with the solid might of the giant engines in his ship—but he had seen explosive charges that he knew would split any ship open from end to end bounce harmlessly from the smooth walls of this ship. He had seen it destroy the fleet of magnetic ships that had formed a supposedly impregnable guard around the mightiest city of Sator.

Then he himself had touched a button, and the giant city had shot off into space, leaving behind it only a screaming tornado and a vast chasm in the crust of the blasted planet.

He could not appreciate the full significance of the velocities Arcot had told him about—he only knew that he had made a bad mistake in underrating the powers of this ship! "I will not touch these things again without your permission, Earthman," Torlos promised earnestly.

The Ancient Mariner drove on through space, rapidly eating up the millions of miles that separated Nansal from Sator. Arcot sat in the control room with Morey discussing their passenger.

"You know," Arcot mused, "I've been thinking about that man's strength; an iron skeleton doesn't explain it all. He has to have muscles to move that skeleton around."

"He's got muscles, all right," Morey grinned. "But I see what you mean; muscles that big should tire easily, and his don't seem to. He seems tireless; I watched him throw those men one after another like bullets from a machine gun. He threw the last one as violently as the first—and those men weighed over three hundred pounds! Apparently his muscles felt no fatigue!"

"There's another thing," pointed out Arcot. "The way he was breathing and the way he seemed to keep so cool. When I got through there, I was dripping with sweat; that hot, moist air was almost too much for me. Our friend? Cool as ever, if not more so.

"And after the fight, he wasn't even breathing heavily!"

"No," agreed Morey. "But did you notice him during the fight? He was breathing heavily, deeply, and swiftly—not the shallow, panting breath of a runner, but deep and full, yet faster than I can breathe. I could hear him breathing in spite of all the noise of the battle."

"I noticed it," Arcot said. "He started breathing before the fight started. A human being can fight very swiftly, and with tremendous vigor, for ten seconds, putting forth his best effort, and only breathe once or twice. For another two minutes, he breathes more heavily than usual. But after that, he can't just slow down back to normal. He has used up the surplus oxygen in his system, and that has to be replaced; he has run into 'oxygen debt'. He has to keep on breathing hard to get back the oxygen surplus his body requires.

"But not Torlos! No fatigue for him! Why? Because he doesn't use the oxygen of the air to do work, and therefore his body is not a chemical engine!"

Morey nodded slowly. "I see what you're driving at. His body uses the heat energy of the air! His muscles turn heat energy into motion the same way our molecular beams do!"

"Exactly—he lives on heat!" Arcot said. "I've noticed that he seems almost cold-blooded; his body is at the temperature of the room at all times. In a sense, he is reptilian, but he's vastly more efficient and greatly different than any reptile Earth ever knew. He eats food, all right, but he only needs it to replace his body cells and to fuel his brain."

"Oh, brother," said Morey softly. "No wonder he can do the things he did! Why, he could have kept up that fight for hours without getting tired! Fatigue is as unknown to him as cold weather. He'd only need sleep to replace worn parts. His world is warm and upright on its axis, so there are no seasons. He couldn't survive in the Arctic, but he's obviously the ideal form of life for the tropics."

As the two men found out later, Morey was wrong on that last point. The men of Torlos' race had a small organ, a mass of cells in the lower abdomen which could absorb food from the bloodstream and oxidize it, yielding heat, whenever the temperature of the blood dropped below a certain point. Then they could live very comfortably in the Arctic zones; they carried their own heaters. Their vast strength was limited then, however, and they were forced to eat more and were more subject to fatigue.

Wade and Fuller had been trying to speak with Torlos telepathically, and had evidently run into difficulty, for Fuller called into the control room: "Hey, Arcot, come here a minute! I thought telepathy was a universal language, but this guy doesn't get our ideas at all! And we can't make out some of his. Just now, he seemed to be thinking of 'nourishment' or 'food', and I found out he was thinking of 'heat'!"

"I'll be right down," Arcot told him, heading for the library.

As he entered, Torlos smiled at him; Arcot picked up his thought easily: "Your friends do not seem to understand my thoughts."

"We are not made as you are," Arcot explained, "and our thought forms are different. To you, 'heat' and 'food' are practically the same thing, but we do not think of them as such."

He continued, explaining carefully to Torlos the differences between their bodies and their methods of using energy.

"Stone bones!" Torlos thought in amazement. "And chemical engines for muscles! No wonder you seem so weak. And yet, with your brains, I would hate to have to fight a war with your people!"

"Which brings me to another point," Arcot continued. "We would like to know how the war between the people of Sator and the people of Nansal began. Has it been going on very long?"

Torlos nodded. "I will tell you the story. It is a history that began many centuries ago; a history of persecution and rebellion. And yet, for all that, I think it an interesting history.

"Hundreds of years ago, on Nansal ..."