"Do you suppose you'll need any help?" he asked simply, realizing that even as he spoke, death on a tomb-world might well be lurking in the background. The question sounded like impulse, but it wasn't. Harwich had lived too long in the shadow of the Forbidden Moon's taunting enigma, not to want to take a personal part in any effort to penetrate its grim secrets. Besides, he had a month's furlough from patrol duty now. The thought of possible adventures to come made his nerves tingle. Paul Arnold's eyes widened. "I almost hoped you would want to join me, Harwich," he stammered happily, seeming only to need the moral support of an experienced spaceman, to bring him out of the black mood he was in. "Shall we go to my laboratory?" * * * * * The Arnold lab and dwelling proved to be one of the oddest that Evan Harwich had ever seen. It was just outside the great steel-ribbed airdrome that confined a warm, breatheable atmosphere over Ganymede City, the small mining metropolis of a dying world. The Arnold lab was a group of subterranean rooms, beneath the desert. They were reached by a private tunnel from the City, and were hermetically sealed against leakage of air to the cold semi-vacuum of the Ganymedean atmosphere above. Cellar rooms, vaults, not exactly modern but restored from some ancient ruin; for Ganymede had had its extinct clans of quasihuman people too, ages ago. A weird place, this was, a place of poverty, perhaps, since all of the Arnold resources must have gone into experimentation; but a homey sort of place, too, with its scatterings of books and quaint art objects and pictures. "This is the Energy Barrage Penetrator, Harwich," Paul Arnold was saying in husky tones, as the two men bent over a copper helix or spiral, attached to a maze of wires, tubes, and power-packs. "I rebuilt it here on this test-block from Dad's plans; with certain rearrangements, of course. But we need a new Gyon condenser, if we want to raise the Penetrator's strength enough to make our venture successful." Evan Harwich nodded beneath the single illuminator bulb that glowed here, its rays glinting from the battered, patched hull of the space ship, RQ257, that stood in the center of the great room, under the airtight exit doors provided for it in the ceiling. "So I see," Harwich commented with subdued eagerness. "Well, that's not so bad. I can buy a new Gyon condenser from one of the supply shops in town. I'm no scientist, fella, but they give us a pretty complete scientific training in the patrol service. Enough so that I can see that the Penetrator is going to do the trick, this time, with your improvements. And I don't think it will take very long to get things ready for a real trip to the Forbidden Moon." The patrol man had hardly finished speaking, when a door, somewhere, groaned on its hinges. In the dusty silence there were footsteps, coming nearer through the series of rooms.