—that this entire closed system was undergoing acceleration—

  —that this therefore was a spaceship, heading rapidly away from the world of Vsiirs and in fact already some ten planetary diameters distant, with the gap growing alarmingly moment by moment—

  —that it would be impossible, even for a Vsiir in metamorphosis, to escape from the spaceship at this point—

  —and that, unless it could persuade the crew of the ship to halt and go back, it would be compelled to undertake a long and dreary voyage to a strange and probably loathsome world, where life would at best be highly inconvenient, and might present great dangers. It would find itself cut off painfully from the rhythm of its own civilization. It would miss the Festival of Changing. It would miss the Holy Eclipse. It would not be able to take part in next spring’s Rising of the Sea. It would suffer in a thousand ways.

  There were six human beings aboard the ship. Extending its perceptors, the Vsiir tried to reach their minds. Though humans had been coming to its planet for many years, it had never bothered making contact with them before; but it had never been in this much trouble before, either. It sent a foggy tendril of thought, roving the corridors, looking for traces of human intelligence. Here? A glow of electrical activity within a sphere of bone: a mind, a mind! A busy mind. But surrounded by a wall, apparently; the Vsiir rammed up against it and was thrust back. That was startling and disturbing. What kind of beings were these, whose minds were closed to ordinary contact? The Vsiir went on, hunting through the ship. Another mind: again closed. Another. Another. The Vsiir felt panic rising. Its mantle fluttered; its energy radiations dropped far down into the visible spectrum, then shot nervously toward much shorter waves. Even its physical form experienced a series of quick involuntary metamorphoses, to the Vsiir’s intense embarrassment. It did not get control of its body until it had passed from spherical to cubical to chaotic, and had become a gridwork of fibrous threads held together only by a pulsing strand of ego. Fiercely it forced itself back to the spherical form and resumed its search of the ship, dismally realizing that by this time its native world was half a stellar unit away. It was without hope now, but it continued to probe the minds of the crew, if only for the sake of thoroughness. Even if it made contact, though, how could it communicate the nature of its plight, and even if it communicated, why would the humans be disposed to help it? Yet it went on through the ship. And—

  Here: an open mind. No wall at all. A miracle! The Vsiir rushed into close contact, overcome with joy and surprise, pouring out its predicament. Please listen. Unfortunate nonhuman organism accidentally transported into this vessel during loading of cargo. Metabolically and psychologically unsuited for prolonged life on Earth. Begs pardon for inconvenience, wishes prompt return to home planet recently left, regrets disturbance in shipping schedule but hopes that this large favor will not prove impossible to grant. Do you comprehend my sending? Unfortunate nonhuman organism accidentally transported—

  ***

  LIEUTENANT FALKIRK HAD DRAWN THE first sleep-shift after floatoff. It was only fair; Falkirk had knocked himself out processing the cargo during the loading stage, slapping the floater-nodes on every crate and feeding the transit manifests to the computer. Now that the ship was spaceborne he could grab some rest while the other crewmen were handling the floatoff chores. So he settled down for six hours in the cradle as soon as they were on their way. Below him, the ship’s six gravity-drinkers spun on their axes, gobbling inertia and pushing up the acceleration, and the ship floated Earthward at a velocity that would reach the galactic level before Falkirk woke. He drifted into drowsiness. A good trip: enough greenfire bark in the hold to see Earth through a dozen fits of the molecule plague, and plenty of other potential medicinals besides, along with a load of interesting mineral samples, and—Falkirk slept. For half an hour he enjoyed sweet slumber, his mind disengaged, his body loose.

  Until a dark dream bubbled through his skull.

  Deep purple sunlight, hot and somber. Something slippery tickling the edges of his brain. He lies on a broad white slab in a scorched desert. Unable to move. Getting harder to breathe. The gravity—a terrible pull, bending and breaking him, ripping his bones apart. Hooded figures moving around him, pointing, laughing, exchanging blurred comments in an unknown language. His skin melting and taking on a new texture: porcupine quills sprouting inside his flesh and forcing their way upward, poking out through every pore. Points of fire all over him. A thin scarlet hand, withered fingers like crab claws, hovering in front of his face. Scratching. Scratching. Scratching. His blood running among the quills, thick and sluggish. He shivers, struggling to sit up—lifts a hand, leaving pieces of quivering flesh stuck to the slab—sits up—

  Wakes, trembling, screaming.

  Falkirk’s shout still sounded in his own ears as his eyes adjusted to the light. Lieutenant Commander Rodriguez was holding his shoulders and shaking him.

  “You all right?”

  Falkirk tried to reply. Words wouldn’t come. Hallucinatory shock, he realized, as part of his mind attempted to convince the other part that the dream was over. He was trained to handle crises; he ran through a quick disciplinary countdown and calmed himself, though he was still badly shaken. “Nightmare,” he said hoarsely. “A beauty. Never had a dream with that kind of intensity before.”

  Rodriguez relaxed. Obviously he couldn’t get very upset over a mere nightmare. “You want a pill?”

  Falkirk shook his head. “I’ll manage, thanks.”

  But the impact of the dream lingered. It was more than an hour before he got back to sleep, and then he fell into a light, restless doze, as if his mind were on guard against a return of those chilling fantasies. Fifty minutes before his programmed wake-up time, he was awakened by a ghastly shriek from the far side of the cabin.

  Lieutenant Commander Rodriguez was having a nightmare.

  ***

  WHEN THE SHIP MADE FLOATDOWN on Earth a month later it was, of course, put through the usual decontamination procedures before anyone or anything aboard it was allowed out of the starport. The outer hull got squirted with sealants designed to trap and smother any microorganism that might have hitchhiked from another world; the crewmen emerged through the safety pouch and went straight into a quarantine chamber without being exposed to the air; the ship’s atmosphere was cycled into withdrawal chambers, where it underwent a thorough purification, and the entire interior of the vessel received a six-phase sterilization, beginning with fifteen minutes of hard vacuum and ending with an hour of neutron bombardment.

  These procedures caused a certain degree of inconvenience for the Vsiir. It was already at the low end of its energy phase, due mainly to the repeated discouragements it had suffered in its attempts to communicate with the six humans. Now it was forced to adapt to a variety of unpleasant environments with no chance to rest between changes. Even the most adaptable of organisms can get tired. By the time the starport’s decontamination team was ready to certify that the ship was wholly free of alien life-forms, the Vsiir was very, very tired indeed.

  The oxygen-nitrogen atmosphere entered the hold once more. The Vsiir found it quite welcome, at least in contrast to all that had just been thrown at it. The hatch was open; stevedores were muscling the cargo crates into position to be floated across the field to the handling dome. The Vsiir took advantage of this moment to extrude some legs and scramble out of the ship. It found itself on a broad concrete apron, rimmed by massive buildings. A yellow sun was shining in a blue sky; infrared was bouncing all over the place, but the Vsiir speedily made arrangements to deflect the excess. It also compensated immediately for the tinge of ugly hydrocarbons in the atmosphere, for the frightening noise level, and for the leaden feeling of homesickness that suddenly threatened its organic stability at the first sight of this unfamiliar, disheartening world. How to get home again? How to make contact, even? The Vsiir sensed nothing but closed minds—sealed like seeds in their shells. True, from time to time the minds of these humans opened, but even then they seemed unwilling to let the Vsiir’s message get through.

  Perhaps it would be different here. Perhaps those six were poor communicators, for some reason, and there would be more receptive minds available in this place. Perhaps. Perhaps. Close to despair, the Vsiir hurried across the field and slipped into the first building in which it sensed open minds. There were hundreds of humans in it, occupying many levels, and the open minds were widely scattered. The Vsiir located the nearest one and, worriedly, earnestly, hopefully, touched the tip of its mind to the human’s. Please listen, I mean no harm. Am nonhuman organism arrived on your planet through unhappy circumstances, wishing only quick going back to own world.

  ***

  THE CARDIAC WING OF LONG Island Starport Hospital was on the ground floor, in the rear, where the patients could be given floater therapy without upsetting the gravitational ratios of the rest of the building. As always, the hospital was full—people were always coming in sick off starliners, and most of them were hospitalized right at the starport for their own safety—and the cardiac wing had more than its share. At the moment it held a dozen infarcts awaiting implant, nine postimplant recupes, five coronaries in emergency stasis, three ventricle-regrowth projects, an aortal patch job, and nine or ten assorted other cases. Most of the patients were floating, to keep down the gravitational strain on their damaged tissues—all but the regrowth people, who were under full Earthnorm gravity so that their new hearts would come in with the proper resilience and toughness. The hospital had a fine reputation and one of the lowest mortality rates in the hemisphere.

  Losing two patients the same morning was a shock to the entire staff.

  At 0917 the monitor flashed the red light for Mrs. Maldonado, 87, postimplant and thus far doing fine. She had developed acute endocarditis coming back from a tour of the Jupiter system; at her age there wasn’t enough vitality to sustain her through the slow business of growing a new heart with a genetic prod, but they’d given her a synthetic implant and for two weeks it had worked quite well. Suddenly, though, the hospital’s control center was getting a load of grim telemetry from Mrs. Maldonado’s bed: valve action zero, blood pressure zero, respiration zero, pulse zero, everything zero, zero, zero. The EEG tape showed a violent lurch—as though she had received some abrupt and intense shock—followed by a minute or two of irregular action, followed by termination of brain activity. Long before any hospital personnel had reached her bedside, automatic revival equipment, both chemical and electrical, had gone to work on the patient, but she was beyond reach: a massive cerebral hemorrhage, coming totally without warning, had done irreversible damage.

  At 0928 came the second loss: Mr. Guinness, 51, three days past surgery for a coronary embolism. The same series of events. A severe jolt to the nervous system, an immediate and fatal physiological response. Resuscitation procedures negative. No one on the staff had any plausible explanation for Mr. Guinness’ death. Like Mrs. Maldonado, he had been sleeping peacefully, all vital signs good, until the moment of the fatal seizure.

  “As though someone had come up and yelled boo in their ears,” one doctor muttered, puzzling over the charts. He pointed to the wild EEG track. “Or as if they’d had unbearably vivid nightmares and couldn’t take the sensory overload. But no one was making noise in the ward. And nightmares aren’t contagious.”

  ***

  DR. PETER MOOKHERJI, RESIDENT IN neuropathology, was beginning his morning rounds on the hospital’s sixth level when the soft voice of his annunciator, taped behind his left ear, asked him to report to the quarantine building immediately. Dr. Mookherji scowled. “Can’t it wait? This is my busiest time of day, and—”

  “You are asked to come at once.”

  “Look, I’ve got a girl in a coma here, due for her teletherapy session in fifteen minutes, and she’s counting on seeing me. I’m her only link to the world. If I’m not there when—”

  “You are asked to come at once, Dr. Mookherji.”

  “Why do the quarantine people need a neuropathologist in such a hurry? Let me take care of the girl, at least, and in forty-five minutes they can have me.”

  “Dr. Mookherji—”

  It didn’t pay to argue with a machine. Mookherji forced his temper down. Short tempers ran in his family, along with a fondness for torrid curries and a talent for telepathy. Glowering, he grabbed a data terminal, identified himself, and told the hospital’s control center to reprogram his entire morning schedule. “Build in a half-hour postponement somehow,” he snapped. “I can’t help it—see for yourself. I’ve been requisitioned by the quarantine staff.” The computer was thoughtful enough to have a rollerbuggy waiting for him when he emerged from the hospital. It whisked him across the starport to the quarantine building in three minutes, but he was still angry when he got there. The scanner at the door ticked off his badge and one of the control center’s innumerable voice-outputs told him solemnly, “You are expected in Room 403, Dr. Mookherji.”

  Room 403 turned out to be a two-sector interrogation office. The rear sector of the room was part of the building’s central quarantine core, and the front sector belonged to the public-access part of the building, with a thick glass wall in between. Six haggard-looking spacemen were slouched on sofas behind the wall, and three members of the starport’s quarantine staff paced about in the front. Mookherji’s irritation ebbed when he saw that one of the quarantine men was an old medical-school friend, Lee Nakadai. The slender Japanese was a year older than Mookherji—29 to 28; they met for lunch occasionally at the starport commissary, and they had double-dated a pair of Filipina twins earlier in the year, but the pressure of work had kept them apart for months. Nakadai got down to business quickly now: “Pete, have you ever heard of an epidemic of nightmares?”

  “Eh?”

  Indicating the men behind the quarantine wall, Nakadai said, “These fellows came in a couple of hours ago from Norton’s Star. Brought back a cargo of greenfire bark. Physically they check out to five decimal places, and I’d release them except for one funny thing. They’re all in a bad state of nervous exhaustion, which they say is the result of having had practically no sleep during their whole monthlong return trip. And the reason for that is that they were having nightmares—every one of them—real mind-wrecking dreams, whenever they tried to sleep. It sounded so peculiar that I thought we’d better run a neuropath checkup, in case they’ve picked up some kind of cerebral infection.”

  Mookherji frowned. “For this you get me out of my ward on emergency requisition, Lee?”

  “Talk to them,” Nakadai said. “Maybe it’ll scare you a little.”

  Mookherji glanced at the spacemen. “All right,” he said. “What about these nightmares?”

  A tall, bony-looking officer who introduced himself as Lieutenant Falkirk said, “I was the first victim—right after floatoff. I almost flipped. It was like, well, something touching my mind, filling it with weird thoughts. And everything absolutely real while it was going on—I thought I was choking, I thought my body was changing into something alien, I felt my blood running out my pores—” Falkirk shrugged. “Like any sort of bad dream, I guess, only ten times as vivid. Fifty times. A few hours later Lieutenant Commander Rodriguez had the same kind of dream. Different images, same effect. And then, one by one, as the others took their sleep-shifts, they started to wake up screaming. Two of us ended up spending three weeks on happy-pills. We’re pretty stable men, doctor—we’re trained to take almost anything. But I think a civilian would have cracked up for good with dreams like those. Not so much the images as the intensity, the realness of them.”

  “And these dreams recurred, throughout the voyage?” Mookherji asked.

  “Every shift. It got so we were afraid to doze off, because we knew the devils would start crawling through our heads when we did. Or we’d put ourselves real down on sleeper-tabs. And even so we’d have the dreams, with our minds doped to a level where you wouldn’t imagine dreams would happen. A plague of nightmares, doctor. An epidemic.”

  “When was the last episode?”

  ‘The final sleep-shift before floatdown.”

  “You haven’t gone to sleep, any of you, since leaving ship?”

  ‘No,” Falkirk said.

  One of the other spacemen said, “Maybe he didn’t make it clear to you, doctor. These were killer dreams. They were mind-crackers. We were lucky to get home sane. If we did.”

  Mookherji drummed his fingertips together, rummaging through his experience for some parallel case. He couldn’t find any. He knew of mass hallucinations, plenty of them, episodes in which whole mobs had persuaded themselves they had seen gods, demons, miracles, the dead walking, fiery symbols in the sky. But a series of hallucinations coming in sequence, shift after shift, to an entire crew of tough, pragmatic spacemen? It didn’t make sense.

  Nakadai said, “Pete, the men had a guess about what might have done it to them. Just a wild idea, but maybe—”

  “What is it?”

  Falkirk laughed uneasily. “Actually, it’s pretty fantastic, doctor.”

  “Go ahead.”

  “Well, that something from the planet came aboard the ship with us. Something, well, telepathic. Which fiddled around with our minds whenever we went to sleep. What we felt as nightmares was maybe this thing inside our heads.”

  “Possibly it rode all the way back to Earth with us,” another spaceman said. “It could still be aboard the ship. Or loose in the city by now.”

  “The Invisible Nightmare Menace?” Mookherji said, with a faint smile. “I doubt that I can buy that.”

  “There are telepathic creatures,” Falkirk pointed out.

  “I know,” Mookherji said sharply. “I happen to be one myself.”

  “I’m sorry, doctor, if—”

  “But that doesn’t lead me to look for telepaths under every bush. I’m not ruling out your alien menace, mind you. But I think it’s a lot more likely that you picked up some kind of inflammation of the brain out there. A virus disease, a type of encephalitis that shows itself in the form of chronic hallucinations.” The spacemen looked troubled. Obviously they would rather be victims of an unknown monster preying on them from outside than of an unknown virus lodged in their brains. Mookherji went on, “I’m not saying that’s what it is, either. I’m just tossing around hypotheses. We’ll know more after we’ve run some tests.” Checking his watch, he said to Nakadai, “Lee, there’s not much more I can find out right now, and I’ve got to get back to my patients. I want these fellows plugged in for the full series of neuropsychological checkouts. Have the outputs relayed to my office as they come in. Run the tests in staggered series and start letting the men go to sleep, two at a time, after each series—I’ll send over a technician to help you rig the telemetry. I want to be notified immediately if there’s any nightmare experience.”