He listened in the silence of the Exploration ship’s control room. He heard nothing but that was what bothered him; an ominous quiet when there should have been a multitude of sounds from the nearby village for the viewscreen’s audio-pickups to transmit. And it was more than six hours past the time when the native, Throon, should have come to sit with him outside the ship as they resumed the laborious attempt to learn each other’s language.

[Illustration: Was the cat native to the planet, or to his imagination?]

The viewscreen was black in the light of the control room, even though it was high noon outside. The dull red sun was always invisible through the world’s thick atmosphere and to human eyes full day was no more than a red-tinged darkness.

He switched on the ship’s outside floodlights and the viewscreen came to bright white life, showing the empty glades reaching away between groves of purple alien trees. He noticed, absently, that the trees seemed to have changed a little in color since his arrival.

The village was hidden from view by the outer trees but there should have been some activity in the broad area visible to him. There was none, not even along the distant segment of what should have been a busy road. The natives were up to something and he knew, from hard experience on other alien worlds, that it would be nothing good. It would be another misunderstanding of some kind and he didn’t know enough of their incomprehensible language to ask them what it was--

Suddenly, as it always came, he felt someone or something standing close behind him and peering over his shoulder. He dropped his hand to the blaster he had taken to wearing at all times and whirled.

Nothing was behind him. There never was. The control room was empty, with no hiding place for anything, and the door was closed, locked by the remote-control button beside him. There was nothing.

The sensation of being watched faded, as though the watcher had withdrawn to a greater distance. It was perhaps the hundredth time within six days that he had felt the sensation. And when he slept at night something came to nuzzle at his mind; faceless, formless, utterly alien. For the past three nights he had not let the blaster get beyond quick reach of his hand, even when in bed.

But whatever it was, it could not be on the ship. He had searched the ship twice, a methodical compartment-by-compartment search that had found nothing. It had to be the work of the natives from outside the ship. Except...

Why, if the natives were telepathic, did the one called Throon go through the weary pretense of trying to learn a mutually understandable form of communication?

There was one other explanation, which he could not accept: that he was following in the footsteps of Will Garret of Ship Nine who had deliberately gone into a white sun two months after the death of his twin brother.

He looked at the chair beside his own, Johnny’s chair, which would forever be empty, and his thoughts went back down the old, bitter paths. The Exploration Board had been wrong when they thought the close bond between identical twins would make them the ideal two-man crews for the lonely, lifetime journeys of the Exploration Ships. Identical twins were too close; when one of them died, the other died in part with him.

They had crossed a thousand light-years of space together, he and Johnny, when they came to the bleak planet that he would name Johnny’s World. He should never have let Johnny go alone up the slope of the honey-combed mountain--but Johnny had wanted to take the routine record photographs of the black, tiger-like beasts which they had called cave cats and the things had seemed harmless and shy, despite their ferocious appearance.

“I’m taking them a sack of food that I think they might like,” Johnny had said. “I want to try to get some good close-up shots of them.”

Ten minutes later he heard the distant snarl of Johnny’s blaster. He ran up the mountainside, knowing already that he was too late. He found two of the cave cats lying where Johnny had killed them. Then he found Johnny, at the foot of a high cliff. He was dead, his neck broken by the fall. Scattered all around him from the torn sack was the food he had wanted to give to the cats.

He buried Johnny the next day, while a cold wind moaned under a lead-gray sky. He built a monument for him; a little mound of frosty stones that only the wild animals would ever see--

A chime rang, high and clear, and the memories were shattered. The orange light above the hyperspace communicator was flashing; the signal that meant the Exploration Board was calling him from Earth.

He flipped the switch and said, “Paul Jameson, Exploration Ship One.”

The familiar voice of Brender spoke:

“It’s been some time since your preliminary report. Is everything all right?”

“In a way,” he answered. “I was going to give you the detailed report tomorrow.”

“Give me a brief sketch of it now.”

“Except for their short brown fur, the natives are humanoid in appearance. But there are basic differences. Their body temperature is cool, like their climate. Their vision range is from just within the visible red on into the infrared. They’ll shade their eyes from the light of anything as hot as boiling water but they’ll look square into the ship’s floodlights and never see them.”

“And their knowledge of science?” Brender asked.

“They have a good understanding of it, but along lines entirely different from what our own were at their stage of development. For example: they power their machines with chemicals but there is no steam, heat, or exhaust.”

“That’s what we want to find--worlds where branches of research unknown to our science are being explored. How about their language?”

“No progress with it yet.” He told Brender of the silence in the village and added, “Even if Throon should show up I could not ask him what was wrong. I’ve learned a few words but they have so many different definitions that I can’t use them.”

“I know,” Brender said. “Variable and unrelated definitions, undetectable shades of inflection--and sometimes a language that has no discernibly separate words. The Singer brothers of Ship Eight ran into the latter. We’ve given them up as lost.”

“The Singers--dead?” he exclaimed. “Good God--it’s been only a month since the Ramon brothers were killed.”

“The circumstances were similar,” Brender said. “They always are. There is no way the Exploration men can tell the natives that they mean them no harm and the suspicion of the natives grows into dangerous hostility. The Singers reported the natives on that world to be both suspicious and possessing powerful weapons. The Singers were proceeding warily, their own weapons always at hand. But, somehow, the natives caught them off-guard--their last report was four months ago.”

There was a silence, then Brender added, “Their ship was the ninth--and we had only fifteen.”

He did not reply to the implications of Brender’s statement. It was obvious to them all what the end of the Plan would be. What it had to be.

It had been only three years since the fifteen heavily armed Exploration ships set out to lead the way for Terran expansion across the galaxy; to answer a cry from far planets, and to find all the worlds that held intelligent life. That was the ultimate goal of the Plan: to accumulate and correlate all the diverse knowledge of all the intelligent life-forms in the galaxy. Among the achievements resulting from that tremendous mass of data would be a ship’s drive faster even than hyperspace; the Third Level Drive which would bring all the galaxies of the universe within reach.

And now nine ships were gone out of fifteen and nineteen men out of thirty...

“The communication barrier,” Brender said. “The damned communication barrier has been the cause behind the loss of every ship. And there is nothing we can do about it. We’re stymied by it...”

The conversation was terminated shortly afterward and he moved about the room restlessly, wishing it was time to lift ship again. With Johnny not there the dark world was like a smothering tomb. He would like to leave it behind and drive again into the star clouds of the galaxy; drive on and on into them--

A ghostly echo touched his mind; restless, poignantly yearning. He swung to face the locked door, knowing there could be nothing behind it. The first real fear came to him as he did so. The thing was lonely--the thing that watched him was as lonely as he was...

What else could any of it be but the product of a mind in the first stage of insanity?

The natives came ten minutes later.

The viewscreen showed their chemically-powered vehicle emerge from the trees and roll swiftly across the glade. Four natives were in it while a fifth one lay on the floor, apparently badly injured.

The vehicle stopped a short distance in front of the airlock and he recognized the native on the floor. It was Throon, the one with whom he had been exchanging language lessons.

They were waiting for him when he emerged from the ship, pistol-like weapons in their belts and grim accusation in their manner.

Throon was muttering unintelligibly, unconscious. His skin, where not covered by the brown fur, was abnormal in appearance. He was dying.

The leader of the four indicated Throon and said in a quick, brittle voice: “Ko reegar feen no-dran!“

Only one word was familiar: Ko, which meant “you” and “yesterday” and a great many other things. The question was utterly meaningless to him.

He dropped his hand a little nearer his blaster as the leader spoke again; a quick succession of unknown words that ended with a harshly demanding “kreson!“

Kreson meant “now,” or “very quickly.” All the other words were unfamiliar to him. They waited, the grim menace about them increasing when he did not answer. He tried in vain to find some way of explaining to them he was not responsible for Throon’s sickness and could not cure it.

Then he saw the spray of leaves that had caught on the corner of the vehicle when it came through the farther trees.

They were of a deep purple color. All the trees around the ship were almost gray by contrast.

Which meant that he was responsible for Throon’s condition.

The cold white light of the ship’s floodlights, under which he and Throon had sat for day after day, contained radiations that went through the violet and far into the ultraviolet. To the animal and vegetable life of the dark world such radiations were invisibly short and deadly.

Throon was dying of hard-radiation sickness.

It was something he should have foreseen and avoided--and that would not have happened had he accepted old Throon’s pantomimed invitation, in the beginning, to go with him into the village to work at the language study. There he would have used a harmless battery lamp for illumination ... but there was no certainty that the natives were not planning to lay a trap for him in the village and he had refused to go.

It did not matter--there was a complex radiation-neutralizer and cell-reconstructor in the ship which would return Throon to full, normal health a few hours after he was placed in its chamber.

He turned to the leader of the four natives and motioned from Throon to the airlock. “Go--there,” he said in the native language.

“Bron!“ the leader answered. The word meant “No” and there was a determination in the way he said it that showed he would not move from it.

At the end of five minutes his attempts to persuade them to take Throon into the ship had increased their suspicion of his motives to the point of critical danger. If only he could tell them why he wanted Throon taken into the ship ... But he could not and would have to take Throon by first disposing of the four without injuring them. This he could do by procuring one of the paralyzing needle-guns from the ship.

He took a step toward the ship and spoke the words that to the best of his knowledge meant: “I come back.”

“Feswin ilt k’la.“

Their reply was to snatch at their weapons in desperate haste, even as the leader uttered a hoarse word of command. He brought up the blaster with the quick motion that long training had perfected and their weapons were only half drawn when his warning came:

“Bron!“

They froze, but did not release their weapons. He walked backward to the airlock, his blaster covering them, the tensely waiting manner in which they watched his progress telling him that the slightest relaxation of his vigilance would mean his death. He did not let the muzzle of the blaster waver until he was inside the airlock and the outer door had slid shut.

He was sure that the natives would be gone when he returned. And he was sure of another thing: That whatever he had said to them, it was not what he had thought he was saying.

He saw that the glade was empty when he opened the airlock again. At the same time a bomb-like missile struck the ship just above the airlock and exploded with a savage crash. He jabbed the Close button and the door clicked shut barely in advance of three more missiles which hammered at its impervious armor.

So that, he thought wearily, is that.

He laid the useless needle-gun aside. The stage was past when he could hope to use it. He could save Throon only by killing some of the others--or he could lift ship and leave Throon to die. Either action would make the natives hate and fear Terrans; a hatred and fear that would be there to greet all future Terran ships.

That was not the way a race gave birth to peaceful galactic empire, was not the purpose behind the Plan. But always, wherever the Exploration men went, they encountered the deadly barrier; the intangible, unassailable communication barrier. With the weapons an Exploration man carried in his ship he had the power to destroy a world--but not the power to ask the simple questions that would prevent fatal misunderstandings.