Hilary pressed the secret spring he knew of. A gleaming oblong of crystal slid silently open. He went in without hesitation and the slide closed with a little whir behind him. A low tunnel confronted him, just barely high enough for him to move without stooping. The walls here were of burnished metal, glowing with impregnated cold-light. It was empty, silent. Evidently it had been undisturbed for years. The Mercutians had not discovered this secret way then. * * * * * The tunnel slanted downward for several hundred yards, then turned sharply upward until a vita-crystal wall barred the way. Hilary could hear vague sounds from the other side. He was in the Robbins Building. He turned to the left, where a shaft stretched upward, completely enclosed by crystal walls. A thin oblong edging showed the platform beneath. He stepped on it, hesitated for a moment. There were two control buttons; one that stopped the lift in the laboratory, the other in the sleeping room that once was Martin Robbins'. Hilary decided in favor of the penthouse; there was less chance of a present occupant of the room. If there was--he shrugged his shoulders and loosened the automatic in his blouse. He pressed the button. The platform shot smoothly upward, up, up, thrusting a thousand feet up. At length it came to a gliding halt. Hilary knew he was on the roof now, in the interior of the wall making one side of the sleep-apartment. The vita-crystal gleamed mockingly opaque at him. If only he could see through; if only he had a Mercutian search beam now. Was there someone in the room on the other side of the wall? He strained his ears to listen, but the crystal was pretty much sound-proof. Very quietly Hilary drew his gun, broke it, examined the chamber. The six bullets lay snug. He snapped it back in position, held the automatic butt against his side, reached over and pressed the release button. * * * * * The slide whirred open. Hilary waited a second, tense, ready to shoot at the slightest sound. His eyes bored through the oblong. Nothing was in sight except the luxurious furnishings he remembered so well; nothing stirred. But his vision was limited to that part of the room framed by the slide. With infinite caution he peered out, his searching gaze flicking swiftly, around the sleep-apartment. It was a man's room with built in divans, automatic sleep-spray, wall rack to hold illuminated book sheets, magnified so as to be read comfortably from a reclining position on the divan--in short, the usual ordered luxuries of a well-furnished sleep-room. It was empty--but the divan was touseled, certain small things disarranged. Someone used this room. Hilary stepped out, leaving the slide behind him open in case of an enforced retreat. He paused to think. Where could Joan be held prisoner--if, and it was a big if--she were really here. He ran over the possibilities.