(There was high adventure on the moon when it was first colonized. Men faced various ways of dying—all of them unpleasant—and found that simply staying alive was a great satisfaction and a full-time occupation. Because of this spirit—which is that of true adventure—there came to be bases where hydroponic gardens freshened the air and men took continued living as a matter of course. This, obviously, was not adventure. So problems arose. Men began to be moved by other motives than the zest they'd known at first. But there were still a great many ways of getting killed on the moon. So there came a time when Colonel Ed McCauley had to insist that certain men under his command put first things first, as adventurers do, and not act for the gratification of their problem personalities.)

Traveling at moon gait, which is the standard travel pace on Earth's big moon, McCauley had ten of the last twenty miles behind him when he saw the sledge trail in the dust. He frowned at it and looked over to the west. He saw Earth, blue-green and glamorous, hanging as usual in the lunar sky just above the edges of the ring mountains. But Earth was always just there. He squinted at the sun through the faceplate of his helmet. It was a trifle over ten degrees above the horizon and it moved across the black, star-speckled sky at half a degree per hour. In twenty hours, then, lunar night would fall. And here was the sledge track that said that the relay unit for Repeater Two, carrying word to and from Farside and the rest of the human race, had passed this way en route to be set up; but the lack of returning footprints said that the men with it had not come back.

Repeater One was already in place and ready to operate. Repeaters Three and Four had also been put in position by men from faraway Farside Base. Repeater Two was necessary to bring Farside Base into communication with the rest of the cosmos. Two weeks of lunar night with no word from outside the base and not even Earth to look at in the sky—this would not be good for the men on Farside.

McCauley stopped. He'd been moving in that swooping, semi-flying fashion which the lesser lunar gravity allows. He stared at the trail. No, the men had not come back. Yet he'd ordered a party of two to set up the relay unit. It was to be put into place on the very tip of a mountain that was now away below the horizon. There it would be in line of sight of Repeater One, which was relatively near, and Repeater Three, which was farther away but which in turn could relay signals to Four, which was farthest away of all. From Four, the relayed messages would go on to Farside Base. When all this was accomplished, the Grimaldi Base ten miles distant could communicate with Farside through Repeaters One, Two, Three, and Four, and with Earth by line-of-sight transmission; so Farside could communicate with Earth and through Earth Relay with all the other moon bases—in short, with all humanity. But Two should have been up and in operation by now.

McCauley shook his head impatiently inside his space helmet. He'd been away from his command for thirty hours, during which he'd traveled twenty miles on foot, at moon gait, to Gerritson Bay. It wasn't a bay, of course, but an intrusion of now-frozen lava into the mountainous country here at the edge of the moon's earthside surface. He'd been met by a moon jeep and had traveled seven hundred miles over a mare—one of the dark areas that were once thought to be seas but actually were dry and level—to the main lunar base near Hipparchus. He'd had a one-hour conference with the base commander there, trying to work out something to prevent the first murder on Earth's big satellite. The conference was unsatisfactory. He'd come back to Gerritson Bay and now he'd covered ten of the twenty remaining miles to Grimaldi Base. When he reached Grimaldi the excessively irritating problem of a murder in the making was still unsolved, and now in addition there was the failure to complete placing the relay at the site of Repeater Two. The sledge ought to be in its place on the peak which was invisible from here, and the men who'd set it up should have returned. They hadn't.

He flipped on his space radio and said curtly:

"McCauley calling relay placing party. Come in!"

There was no answer. He called again and again. Then he called Grimaldi Base. Again no answer. He was out of radio contact with all humanity on the moon—even his own base ten miles away—though by switching frequencies he could raise Earth Relay a quarter million miles farther away. The men with the moon sledge might only be behind a mountain wall or anywhere in any direction below the horizon, but radio communication on the moon is limited to line-of-sight because there is no air and hence no layer of ions to bounce radio signals down behind obstacles or around the moon's curvature.

McCauley started off again, fuming. Moon gait is a highly specialized form of travel. In one-sixth gravity a man can cover ten miles an hour over rough ground if he knows the trick of the gait and the trail is marked. He travels in slow-motion giant steps, with something of the effect of an extremely deliberate ballet. He begins with a leap up and forward, and he rises slowly and deliberately while soaring ahead. At mid-leap he is six feet higher than at take-off. Then he descends slowly and with dignity, touches ground and strides at the same time, and bounds up and ahead once more. There are long seconds between steps and long yards between strides. When a person is used to it, moon gait is almost restful. Some people even find it familiar. They've dreamed of such effortless half flight in their sleep.

Now, though he was disturbed, McCauley made two miles with no other known cause for worry than the lateness of the two men who'd placed the relay and the prospective killing he'd had on his mind before. He passed between precipices and over dust-strewn stone and through winding defiles. The two men should be back....