The letter came down the slot too early that morning to be the regular mail run. Pete Greenwood eyed the New Philly photocancel with a dreadful premonition. The letter said:

PETER:
Can you come East chop-chop, urgent?
Grdznth problem getting to be a PRoblem, need expert
icebox salesman to get gators out of hair fast.
Yes? Math boys hot on this, citizens not so hot.
Please come.
TOMMY

Pete tossed the letter down the gulper with a sigh. He had lost a bet to himself because it had come three days later than he expected, but it had come all the same, just as it always did when Tommy Heinz got himself into a hole.

Not that he didn’t like Tommy. Tommy was a good PR-man, as PR-men go. He just didn’t know his own depth. PRoblem in a beady Grdznth eye! What Tommy needed right now was a Bazooka Battalion, not a PR-man. Pete settled back in the Eastbound Rocketjet with a sigh of resignation.

He was just dozing off when the fat lady up the aisle let out a scream. A huge reptilian head had materialized out of nowhere and was hanging in air, peering about uncertainly. A scaly green body followed, four feet away, complete with long razor talons, heavy hind legs, and a whiplash tail with a needle at the end. For a moment the creature floated upside down, legs thrashing. Then the head and body joined, executed a horizontal pirouette, and settled gently to the floor like an eight-foot circus balloon.

Two rows down a small boy let out a muffled howl and tried to bury himself in his mother’s coat collar. An indignant wail arose from the fat lady. Someone behind Pete groaned aloud and quickly retired behind a newspaper.

The creature coughed apologetically. “Terribly sorry,” he said in a coarse rumble. “So difficult to control, you know. Terribly sorry...” His voice trailed off as he lumbered down the aisle toward the empty seat next to Pete.

The fat lady gasped, and an angry murmur ran up and down the cabin. “Sit down,” Pete said to the creature. “Relax. Cheerful reception these days, eh?”

“You don’t mind?” said the creature.

“Not at all.” Pete tossed his briefcase on the floor. At a distance the huge beast had looked like a nightmare combination of large alligator and small tyrannosaurus. Now, at close range Pete could see that the “scales” were actually tiny wrinkles of satiny green fur. He knew, of course, that the Grdznth were mammals--”docile, peace-loving mammals,” Tommy’s PR-blasts had declared emphatically--but with one of them sitting about a foot away Pete had to fight down a wave of horror and revulsion.

The creature was most incredibly ugly. Great yellow pouches hung down below flat reptilian eyes, and a double row of long curved teeth glittered sharply. In spite of himself Pete gripped the seat as the Grdznth breathed at him wetly through damp nostrils.

“Misgauged?” said Pete.

The Grdznth nodded sadly. “It’s horrible of me, but I just can’t help it. I always misgauge. Last time it was the chancel of St. John’s Cathedral. I nearly stampeded morning prayer--” He paused to catch his breath. “What an effort. The energy barrier, you know. Frightfully hard to make the jump.” He broke off sharply, staring out the window. “Dear me! Are we going east?”

“I’m afraid so, friend.”

“Oh, dear. I wanted Florida.”

“Well, you seem to have drifted through into the wrong airplane,” said Pete. “Why Florida?”

The Grdznth looked at him reproachfully. “The Wives, of course. The climate is so much better, and they mustn’t be disturbed, you know.”

“Of course,” said Pete. “In their condition. I’d forgotten.”

“And I’m told that things have been somewhat unpleasant in the East just now,” said the Grdznth.

Pete thought of Tommy, red-faced and frantic, beating off hordes of indignant citizens. “So I hear,” he said. “How many more of you are coming through?”

“Oh, not many, not many at all. Only the Wives--half a million or so--and their spouses, of course.” The creature clicked his talons nervously. “We haven’t much more time, you know. Only a few more weeks, a few months at the most. If we couldn’t have stopped over here, I just don’t know what we’d have done.”

“Think nothing of it,” said Pete indulgently. “It’s been great having you.”

The passengers within earshot stiffened, glaring at Pete. The fat lady was whispering indignantly to her seat companion. Junior had half emerged from his mother’s collar; he was busy sticking out his tongue at the Grdznth.

The creature shifted uneasily. “Really, I think--perhaps Florida would be better.”

“Going to try it again right now? Don’t rush off,” said Pete.

“Oh, I don’t mean to rush. It’s been lovely, but--” Already the Grdznth was beginning to fade out.

“Try four miles down and a thousand miles southeast,” said Pete.

The creature gave him a toothy smile, nodded once, and grew more indistinct. In another five seconds the seat was quite empty. Pete leaned back, grinning to himself as the angry rumble rose around him like a wave. He was a Public Relations man to the core--but right now he was off duty. He chuckled to himself, and the passengers avoided him like the plague all the way to New Philly.

But as he walked down the gangway to hail a cab, he wasn’t smiling so much. He was wondering just how high Tommy was hanging him, this time.

The lobby of the Public Relations Bureau was swarming like an upturned anthill when Pete disembarked from the taxi. He could almost smell the desperate tension of the place. He fought his way past scurrying clerks and preoccupied poll-takers toward the executive elevators in the rear.

On the newly finished seventeenth floor, he found Tommy Heinz pacing the corridor like an expectant young father. Tommy had lost weight since Pete had last seen him. His ruddy face was paler, his hair thin and ragged as though chunks had been torn out from time to time. He saw Pete step off the elevator, and ran forward with open arms. “I thought you’d never get here!” he groaned. “When you didn’t call, I was afraid you’d let me down.”

“Me?” said Pete. “I’d never let down a pal.”

The sarcasm didn’t dent Tommy. He led Pete through the ante-room into the plush director’s office, bouncing about excitedly, his words tumbling out like a waterfall. He looked as though one gentle shove might send him yodeling down Market Street in his underdrawers. “Hold it,” said Pete. “Relax, I’m not going to leave for a while yet. Your girl screamed something about a senator as we came in. Did you hear her?”

Tommy gave a violent start. “Senator! Oh, dear.” He flipped a desk switch. “What senator is that?”

“Senator Stokes,” the girl said wearily. “He had an appointment. He’s ready to have you fired.”

“All I need now is a senator,” Tommy said. “What does he want?”

“Guess,” said the girl.

“Oh. That’s what I was afraid of. Can you keep him there?”

“Don’t worry about that,” said the girl. “He’s growing roots. They swept around him last night, and dusted him off this morning. His appointment was for yesterday, remember?”

“Remember! Of course I remember. Senator Stokes--something about a riot in Boston.” He started to flip the switch, then added, “See if you can get Charlie down here with his giz.”

He turned back to Pete with a frantic light in his eye. “Good old Pete. Just in time. Just. Eleventh-hour reprieve. Have a drink, have a cigar--do you want my job? It’s yours. Just speak up.”

“I fail to see,” said Pete, “just why you had to drag me all the way from L.A. to have a cigar. I’ve got work to do.”

“Selling movies, right?” said Tommy.

“Check.”

“To people who don’t want to buy them, right?”

“In a manner of speaking,” said Pete testily.

“Exactly,” said Tommy. “Considering some of the movies you’ve been selling, you should be able to sell anything to anybody, any time, at any price.”

“Please. Movies are getting Better by the Day.”

“Yes, I know. And the Grdznth are getting worse by the hour. They’re coming through in battalions--a thousand a day! The more Grdznth come through, the more they act as though they own the place. Not nasty or anything--it’s that infernal politeness that people hate most, I think. Can’t get them mad, can’t get them into a fight, but they do anything they please, and go anywhere they please, and if the people don’t like it, the Grdznth just go right ahead anyway.”

Pete pulled at his lip. “Any violence?”

Tommy gave him a long look. “So far we’ve kept it out of the papers, but there have been some incidents. Didn’t hurt the Grdznth a bit--they have personal protective force fields around them, a little point they didn’t bother to tell us about. Anybody who tries anything fancy gets thrown like a bolt of lightning hit him. Rumors are getting wild--people saying they can’t be killed, that they’re just moving in to stay.”

Pete nodded slowly. “Are they?”

“I wish I knew. I mean, for sure. The psych-docs say no. The Grdznth agreed to leave at a specified time, and something in their cultural background makes them stick strictly to their agreements. But that’s just what the psych-docs think, and they’ve been known to be wrong.”

“And the appointed time?”

Tommy spread his hands helplessly. “If we knew, you’d still be in L.A. Roughly six months and four days, plus or minus a month for the time differential. That’s strictly tentative, according to the math boys. It’s a parallel universe, one of several thousand already explored, according to the Grdznth scientists working with Charlie Karns. Most of the parallels are analogous, and we happen to be analogous to the Grdznth, a point we’ve omitted from our PR-blasts. They have an eight-planet system around a hot sun, and it’s going to get lots hotter any day now.”

Pete’s eyes widened. “Nova?”

“Apparently. Nobody knows how they predicted it, but they did. Spotted it coming several years ago, so they’ve been romping through parallel after parallel trying to find one they can migrate to. They found one, sort of a desperation choice. It’s cold and arid and full of impassable mountain chains. With an uphill fight they can make it support a fraction of their population.”

Tommy shook his head helplessly. “They picked a very sensible system for getting a good strong Grdznth population on the new parallel as fast as possible. The males were picked for brains, education, ability and adaptability; the females were chosen largely according to how pregnant they were.”

Pete grinned. “Grdznth in utero. There’s something poetic about it.”

“Just one hitch,” said Tommy. “The girls can’t gestate in that climate, at least not until they’ve been there long enough to get their glands adjusted. Seems we have just the right climate here for gestating Grdznth, even better than at home. So they came begging for permission to stop here, on the way through, to rest and parturiate.”

“So Earth becomes a glorified incubator.” Pete got to his feet thoughtfully. “This is all very touching,” he said, “but it just doesn’t wash. If the Grdznth are so unpopular with the masses, why did we let them in here in the first place?” He looked narrowly at Tommy. “To be very blunt, what’s the parking fee?”

“Plenty,” said Tommy heavily. “That’s the trouble, you see. The fee is so high, Earth just can’t afford to lose it. Charlie Karns’ll tell you why.”

Charlie Karns from Math Section was an intense skeleton of a man with a long jaw and a long white coat drooping over his shoulders like a shroud. In his arms he clutched a small black box.

“It’s the parallel universe business, of course,” he said to Pete, with Tommy beaming over his shoulder. “The Grdznth can cross through. They’ve been able to do it for a long time. According to our figuring, this must involve complete control of mass, space and dimension, all three. And time comes into one of the three--we aren’t sure which.”

The mathematician set the black box on the desk top and released the lid. Like a jack-in-the-box, two small white plastic spheres popped out and began chasing each other about in the air six inches above the box. Presently a third sphere rose up from the box and joined the fun.

Pete watched it with his jaw sagging until his head began to spin. “No wires?”

“Strictly no wires,” said Charlie glumly. “No nothing.” He closed the box with a click. “This is one of their children’s toys, and theoretically, it can’t work. Among other things, it takes null-gravity to operate.”

Pete sat down, rubbing his chin. “Yes,” he said. “I’m beginning to see. They’re teaching you this?”

Tommy said, “They’re trying to. He’s been working for weeks with their top mathematicians, him and a dozen others. How many computers have you burned out, Charlie?”

“Four. There’s a differential factor, and we can’t spot it. They have the equations, all right. It’s a matter of translating them into constants that make sense. But we haven’t cracked the differential.”

“And if you do, then what?”

Charlie took a deep breath. “We’ll have inter-dimensional control, a practical, utilizable transmatter. We’ll have null-gravity, which means the greatest advance in power utilization since fire was discovered. It might give us the opening to a concept of time travel that makes some kind of sense. And power! If there’s an energy differential of any magnitude--” He shook his head sadly.

“We’ll also know the time-differential,” said Tommy hopefully, “and how long the Grdznth gestation period will be.”

“It’s a fair exchange,” said Charlie. “We keep them until the girls have their babies. They teach us the ABC’s of space, mass and dimension.”

Pete nodded. “That is, if you can make the people put up with them for another six months or so.”

Tommy sighed. “In a word--yes. So far we’ve gotten nowhere at a thousand miles an hour.”

“I can’t do it!” the cosmetician wailed, hurling himself down on a chair and burying his face in his hands. “I’ve failed. Failed!”