Seth’s reply was a noncommittal snort. We began to move off along a dreary dark-walled corridor. It took us, I knew, along the fastest path to our destination at level five, sector fifty-six. The scenery as we progressed was uninspiring. If anything, it reminded me of the basement levels of a neglected hospital in a run-down area of a large city. There were the same endless corridors, leading to elevators unadorned by any touch of personality. There were rooms and cubicles and overhead pipes and ducts, all color-coded in a way that stamped out all chance of individuality. In saying that I was eager to experience the overall ambience of Sky City, I had lied. Already I had had enough of Sky City.

The fact that the RV helmet could not provide olfactory experiences was probably a blessing. I am exceptionally sensitive to smells, and I felt sure that those around Seth were all unpleasant.

Neither of us chose to speak, and as we went on in total silence I considered Seth’s own probable thought processes. He had come to me from desperation, when his hopes of solving the murders were at lowest ebb. He needed my help; and judging from my recent researches into the Argos Group, he was, like anyone in their senior echelons, willing to do anything to obtain an objective. He would love for me to catch the killer, but he would surely like it better were he able to discover the key clue and solve the case himself.

At the moment, neither of those eventualities appeared probable. Tanya Bishop had been killed on January. 10. It was now the middle of July. We were following a trail that was more than six months old, in an environment where every scent, either literal or metaphorical, was routinely obliterated by the ever-active cleaning machines of Sky City.

I had a random and improbable thought, shocking enough to make me blurt, ’The Sky City cleaning machines are fairly intelligent, aren’t they? Could one be programmed to commit a murder? If it could, that would explain why the victims don’t seem to have been suspicious of the murderer before they were killed.”

The moving scene before me froze; Seth had stopped in his tracks. “I don’t know,” he said at last. “But the cleaning machines are just simple forms of rolfes. Since Gordy Rolfe’s the mastermind behind all of them, an’ he’s a warped little bastard, I assume that the answer is yes. It would need special programs to bypass inhibitor circuits, but you could probably make a cleaning machine-or any other rolfe — kill somebody. So what?”

“So we find out who’s in charge of them. That person would be in the absolute best position to arrange for the killings and still have a perfect alibi.”

Even before Seth’s reply reached me, I saw the fatal flaw in my idea. “Suppose a machine could kill ’em,” he said. “How would it know who to kill, and where to do the killin? If a rolfe hung around one place and splattered anybody who came by, they wouldn’t all be young girls. An’ if the murderer decided who he wanted to kill in advance, a machine would be noticed if it followed her around. And what about the sexual mutilation?”

I could imagine a killer, sufficiently deranged, deriving gratification from the simple knowledge that such an act was being performed; but Seth’s other arguments were unanswerable. The problem was, my question still had validity. Why hadn’t the victims been suspicious of their murderer, particularly after the first few deaths?

It has long been observed that any fool can ask more questions than the wisest man can answer. Seth decided, rightly, that this was one of those cases, though we might disagree as to who was the fool. The image in my visor began to change again. The subject had for the moment been dropped, and soon we were emerging to a totally different and disconcerting environment.

The chamber was gigantic, at least a hundred meters across. I cannot use the terms high or wide, since the space was so close to free fall that it lacked any indicators of preferred direction. I was saved from the possibility of acute discomfort at the sight of the great open arena ahead only by the extraordinary number of curvilinear structures that crisscrossed it in all directions. Most had obvious uses: pleated ducts, anything from a few centimeters to a full meter across, suitable for the transport of bulk materials; silver beams, from their placement employed as structural supports; thin and convoluted branching pipes, holding either optic bundles or serving as pneumatic delivery systems; and delicate-looking silver wires and cables, along which swarmed a variety of multiarmed machines.

I must pause here and seek to articulate why this chamber had an immediate effect on me. In one sense, nothing was new. I had, after all, seen every item in the great room before, albeit not in space and not combined as they were here. Why, then, did the whole induce a profound change in my overall pattern of thought?

During the twenty-seven years since the supernova event, I had known what every thinking being on the planet must recognize: A great disaster lay in the future, worse than the one in the recent past. A deadly particle storm was on the way from Alpha Centauri. It would arrive, as certainly as tomorrow’s sunrise. The space shield was humanity’s best answer.

I knew what was coming, and had made preparations accordingly. The tunneled shelters deep under Otranto Castle contained supplies sufficient for several lifetimes. I and my dear ones would survive, come what may. That effort had been completed years ago, before the youngest six had been born again.

I regarded my efforts as necessary, but clearly not sufficient. The prospect of a long stay in the deep shelters held no appeal, and a superior answer to the problem of survival was a shielding of the whole planet. That called for construction of the space shield, and in turn a nerve center and nucleus was needed for that effort: Sky City. The human race embarked on its first — but not, one hoped, its last-worldwide and long-term construction program.

I am, in spite of what my detractors might say, a member of that human race. I am also aware of my own abilities. Why, then, did I not, with survival at the castle ensured, apply my talents to the other and greater issue of species survival?

For three reasons. First, my expertise lies in the field of biological and medical science, which one might argue has little or no value to shield construction work. Second, the thought of flying out to space was like a clammy hand around my heart. Third, and most significant, my fingerprints, retinal prints, and DNA signature are on record. If I ever appear in a situation, anywhere in this world or off it, where a routine ID is called for, my capture and return to long-term judicial sleep are guaranteed.

Given these powerful reasons why I could not involve myself in humanity’s salvation, I had pushed everything connected with the space shield to the periphery of my attention. For more than a decade, news items about Sky City were deleted or skipped by my news analyzer, and headlines on shield progress or problems ignored.

One cannot, however, fail to see what lies literally before one’s eyes. Faced with the evidence presented by the RV helmet, my own shield of deliberate ignorance vanished. I was out in space, Sky City was real, the shield was real, and our disparate destinies had suddenly coalesced.

Need I add that I did not approve of the change?

Meanwhile, Seth moved slowly but confidently through the maze of the chamber and reached a tunnel on the other side. He advanced maybe forty meters, then suddenly halted. It was not because of word or action on my part. It was also, so far as I could see, not because of the intrinsic information content of the scene ahead. We were descending a long, sharp-angled spiral, an empty twisting stair that led from one level to the next.