Alex grasped the chest-high steel rung firmly with both hands, then lifted his right foot to the rusty bottom one. Eyes front! Never look down! Old advice, still valid. He could do this. Acrophobia was all in the mind.

“You can climb!” Kerry's voice was a low but intense whisper, fiercely demanding. He turned his head for a last look at her pain-wracked face, almost within arm’s reach through the tall window. Behind her was shattered glassware where a lab bench had tipped over, by the sprawled body of a uniformed guard. A second soldier, the one he had killed during the desperate struggle for the man’s minirocket pistol, was out of sight.

There was a gray pallor beneath the clear olive of Kerry’s skin; shock was setting in. The tourniquet on the upper left arm had stopped the bleeding, but the slim rocket had caught just enough of the humerus to break the bone. He had rigged a neck sling to support the arm and prevent more damage, but the bone ends could still rub together and cause intense pain.

Alex looked upward to his goal, some sixty feet above and far to the right of this rusted steel fire escape platform. The east wall of this old sea-front castle curved inward, then sharply out again, reaching to within a few feet of the top of the high promontory which the building abutted. The ground there had been flattened to make an adjacent heliport, connected to the roof by a twenty-foot wooden bridge.

This line of steel rungs in front of Alex, each separately anchored in the thick stone wall, curved sharply to the right, reaching the roof close to the bridge. After climbing ten feet, he would be above open space. Since the day he discovered how serious his acrophobia was, at age eight, Alexander Carter had known that someday, in some inevitable way, this would happen. He could not have known then how much would be at stake, or guess the strength of the love and need that drove him.

For most of Alex’s adolescence, Kerry Singleton had been a lean, surprisingly strong girl of his own age, who could beat him in most sports requiring speed and agility. She was a mocking, smart-mouthed, unwanted but frequent companion in classes and on the sports fields. He would never have believed he could come to love this fellow special student enough to die for her.

Alex realized he was stalling -- that he was unable to lift his left foot to that second rung, to start this hazardous climb into the silent darkness. He looked again at Kerry, in time to see the first doubt appear in her dark brown eyes. He had to climb. There was no other way. A soldier in an armored pillbox guarded the elevator on the roof. They had noted this when they were taken in from the helicopter, three days ago. An electronic lock on a wire mesh screen door across the entrance could only be released by the guard at his station. Anyone who could not give the daily password was taken prisoner, searched, and deposited in a tiny holding cell adjacent to the elevator.

The plan Kerry had outlined after their surprise attack succeeded was simple. Alex would climb to just below the rooftop, and wait. Kerry would ride up in the creaky old elevator, at the end of the long hall just outside the lab door. When she gave the wrong password, the guard would have to emerge from his armored pillbox to arrest her and place her in the cell. And once he was in the open, Alex would ease his upper body over the roof edge and shoot him. The only sound would be a low hiss from the long-barreled minirocket pistol tucked snugly in his belt.

But Alex and Kerry were playing roles that should have been reversed. If only he had been as fast as his wife when they attacked the guards. Kerry had smashed one across the bridge of the nose, kicked behind his left knee socket to bring him down, then caught him with a second kick to the head that snapped his neck. Armed soldiers, both strong men in their early forties, they had not been prepared for a sudden bare-handed attack by two nineteen-year olds. It was unimaginable that a slim young woman like Kerry could take either of them in a one-on-one fight. But Alex did not have her lightning speed; the second guard had gotten off one shot at Kerry. Coming in from the side, Alex had chopped desperately at the extended hand, catching it in time to deflect the barrel and send the high-velocity rocket through Kerry’s arm instead of her heart.

Now Alex must make this climb, when it should have been Kerry. And he must do it immediately, because the guard shift changed in less than an hour. The alternative was that they could both expect to die, probably horribly, with surgeons cutting and probing at their healthy organs. The aged dictator of this small Central American country would grind them both into sausage and eat them at breakfast for a year, just to extend his own fading life that long. Like most of the rulers of the world, he had a distorted understanding of the longevity project that had produced Alex and Kerry.

Alex remembered his own surprise and shock, shortly after his fourteenth birthday, when he learned the truth about his carefully controlled heritage and background. He had not known that the woman who tended and cared for him, whom he called “mother,” was a professional nurse . . . the man he thought his father a brilliant young psychologist, for whom raising Alex had become his life’s most important work . . . that the school at an isolated army base in Arizona was designed for the benefit of a small group of very special children, the donor eggs and sperm that created them subtly altered by precise cuts of molecular knives . . . and that, so far, the experiment had failed.

The faces of his surrogate parents appeared in Alex’s mind; grave, sorrowful, heavy with guilt and love. All the special children had been force-fed biology lessons far beyond current grade levels, to help them understand the importance of their own bodies. At age fourteen Kerry and Alex, the two oldest, had provided the first modified eggs and sperm. The geneticists were sure, this time, that they could finally control the wasting away of the telomeres, the protective caps on the ends of all chromosomes. These caps shortened after each division, until a cell stopped dividing entirely after about fifty replications. They were certain, and it was time for the final modifications.

Alex could remember masturbating, ejaculating into endless tubes of carefully prepared buffering solutions, his young body a fountain of cream, a tower of plenty. But at least he could go into the bathroom, to perform this necessary act in private. Kerry had to suffer the indignity, each month, of physical invasion of her body; injected fluids, suction, the search for the precious egg. She ovulated like clockwork, regular as the moon. The doctors were always there, waiting like white-coated wolves at the doors of her privacy; she learned to bear them with unconcealed ill-will.

The plan was two generations to immortality -- or at minimum, life extension for some unknown number of years. The world groaned and staggered beneath the weight of its present population, nine billion and still climbing. That did not stop each individual from wanting to live longer.

Alex Carter and Kerry Singleton were the result of genetic experiments, but they were adults now. The bodies involved should belong only to themselves. But Alex had known and accepted for years that this was not their reality. They were the property of the United States, a huge investment in money, time and talent.

From the time the group of young people were old enough to understand, the scientists had tried to be frank with them. Alex even knew that his first young sperm were immature, incapable of fertilizing an egg. The later ones were better, but still weak. No actual tries at conception occurred until he was sixteen. Then all efforts, in both the complicated in vitro processes and the carefully chosen young adult women they brought to his lonely bed, were failures. Alex held up well through it all. Quantity he had, but the quality was low. They could only wait, and hope he would be a late maturer. Then wait some more, as the swift years of adolescence passed . . .

Kerry had no problems. At age fourteen she was producing perfect eggs, and today five children could have called her “mother,” if they had not been implanted in surrogate wombs. But none of their fathers had the same altered gene pattern as Alex, and all indications were that the boys’ mature spermatocytes would be regrettably normal. Only a combination of genes from the special group, improved once more, could produce the immortal child. And only a study of those second generation genes would open the final door, the knowledge of how to control cell division in living beings; make the secret of life extension available to everyone.

Seven months back, shortly after Alex, younger than Kerry by a few weeks, had turned nineteen, the two had been brought up to date on all failed efforts. Then they had listened to a surprising suggestion. Would they like to try the proven, old-fashioned way nature had devised to produce babies? In short, live together, have sex, and see if unaided sperm and egg, in their natural environment, could do better than the best processes available outside the womb.

“We would like to marry instead,” Kerry had said quietly. And for the first time the psychologist fathers, nurse mothers, guards, scientists, administrators -- all learned that Kerry and Alex had done the unthinkable -- they had fallen in love.

And so the highly experienced Alex had married and repeatedly made love to the virginal Kerry (biological mother of five, hymen surgically removed, body accustomed to the cold feel of stainless probe and thrusting plastic tube, but a virgin all the same) for six months now.

And there was no sign of a pregnancy to date.

There was no doubt at all that Alex was the weak link. But some of the geneticists believed that his spermatocytes were continuing to change and mature, that he might at any time produce viable sperm. They wanted them to keep trying.

The young married couple had contrived to slip away from the ever-present guards during a trip to Flagstaff. They had wanted an hour or two to mix with strangers on a dance floor, drink a beer at a regular bar, look on unfamiliar faces. Such activities had been prohibited for five months, after details on the Project had finally leaked to the intelligence agencies of the world. And they had been kidnapped, obviously by people watching and waiting for them, and brought to this old castle and another set of secret laboratories. Only this small country’s aged dictator refused to believe they could not immediately restore his lost youth and sexual vitality.

Kerry was still staring intently through the window into Alex’s face, silently urging him to start up the wall. She had always been the more determined of the two. But Kerry’s strength of will could not be transferred. Alex was unable to lift his left foot to the second rung, to force himself out over that waiting abyss.

“Alex,” said Kerry, her voice low, but strong and vibrant. “Alex, you have to climb, phobia or not. You must, my love, for me, for us -- because I'm pregnant!”

Alex stared at her, unbelieving; her eyes finally broke contact, as though suddenly shy. “I'm a week overdue,” Kerry went on quietly. “And you know my cycle, regular as a cesium clock. I’m almost certain we’ve finally done it.”

Kerry was pregnant with immortal seed. It was now immeasurably more important that they escape.

The rusty rungs waited. Alex must climb them, or be forever unable to live with his own conscience.

Alex closed his eyes, raised his left foot to the second rung, stepped up, took the next step, and the next, climbing rapidly now, eyelids still pressed tightly together, not stopping to think, to fear. He was almost half-way up the sheer wall before he realized he was making small noises, scrapes and light bumps; enough to be heard on the roof in the quiet night. He opened his eyes, kept his gaze on the wall only inches away, and slowed his speed -- but he kept climbing.

Alex looked up, to see the line of the roof just above his head, and the swinging gate in the guardrail that gave access to the first rung. He stopped, carefully removed the pistol from his belt, and thumbed off the safety. One quick surge would bring him up over the edge. And Alex was an excellent pistol shot.

From a few feet away Alex heard a low scuffing sound, followed by a slight creaking of leather. With a thrill of fear he realized he was hearing slow footsteps, coming toward him. The bored guard, disregarding his duty in these hours when the elevator almost never operated, was out of his pillbox, walking around on the roof.

Alex raised the pistol, eyes straining to see in the dimness, ready to fire if a face appeared above him. He heard the steps come closer, then pause, a few feet to his right. A boot settled on the foot-high bottom rail, and a cigarette lighter flared in the darkness. Seconds later a glowing coal of fire appeared in the air, a face dimly visible behind it. The man was resting both elbows on the upper railing.

The safety barrier was set back six inches from the edge; Alex could barely make out the man’s nose and bearded cheeks. He could not shoot him from this angle. Alex started to push his upper body away from where he clung close to the wall, his right arm with the pistol lifting and swinging out . . . and the guard casually looked down and saw him.

The soldier’s backward step was instantaneous, just in time for the rocket Alex had aimed at his throat to hiss harmlessly by and go flaming off into the sky.

Alex surged upward one rapid step, bringing his shoulders past the edge, the left hand gripping a vertical pipe stanchion, the pistol extended. But the soldier, who had been carrying his rifle in his free hand, had fast reflexes. He swung the barrel against Alex’s hand in time to deflect the nozzle, sending a second tiny rocket hissing off toward the sea. The pistol clattered to the roof. Ignoring the pain in his hand, Alex quickly took another step up and heaved his upper torso above the edge and under the bottom rail of the gate.

Time seemed to slow for Alex, miliseconds become minutes. Now that he was defenseless, the guard had decided not to shoot him. Instead, he was reversing his rifle, swinging it up to grip it by the barrel. Then the butt was coming toward Alex's head as he tried to pull his body forward. He saw the descending steel plate on the end and could not avoid it, could do nothing at all.

Just before the numbing blow, Alex heard the faint thuck! sound of an impact above him.

Kerry stared intently at the rooftop, trying to see what had happened. She had been watching, standing in shadow at one side of the window, and had seen the smoking guard leaning on the rail. The moonlight was too faint for a good shot; she had left him to Alex. But then the guard had looked down, and she quickly raised her pistol. When he reversed the rifle and lifted it to strike with the butt, she knew Alex was helpless. The guard’s head and shoulders were visible, and she took the shot.

As if to draw a curtain on the tragedy of death, a cloud drifted over the old castle, abruptly blotting out the moonlight. She strained through the darkness, trying to determine if the guard was still standing, but could no longer even see the roof.

Kerry had to move fast. The elevator was still the only way for her. She hurried to a large metal cabinet they had explored earlier, where they had found a pair of bolt-cutters. These had figured in an earlier plan, which they had discarded when they could think of no way to cut the mesh screen under the watchful eyes of the rooftop guard.

Kerry tucked the heavy tool through the sling above her wounded arm, ignoring the fresh pain. She hurried through the sliding doors into the hall, pistol ready in her right hand, but saw no one. The elevator was unlocked and unguarded. The soldier always on the roof was considered adequate security. She pressed the sube button with the barrel of the pistol.

The door locked open automatically when the elevator stopped, exposing the riders to the guard in his pillbox. Kerry started cutting through the wire mesh sliding door, the heavy tool awkward using only one hand. She had to set one of its arms on the floor and press down on the other, making a u-shaped hole less than knee-high.

It was several minutes before Kerry was able to bend up a square patch of screen the size of her body. She wriggled through, stood up, and hurried to the section of railing where the fire escape descended.

The small cloud shadowing the castle had passed as Kerry worked. In the soft moonlight she saw a dark figure sprawled on the stone. A second was half under the swinging gate, bent at the waist, lower body dangling over the edge.

Dropping to her knees by Alex, she looked into his blooded face. His eyes were glazed, but Alex was semi-conscious. Kerry could not drag him fully on the roof with one arm. She spoke softly to him, then lightly slapped his face when there was no reaction. He lay still. She grasped a handful of hair and gave it a vicious yank. There was a grunt of pain, and she yanked another section. She saw his mouth open in protest, ready to yell, and she quickly clapped her hand over his lips.

“Alex! Wake up! Pull yourself the rest of the way through!”

Full consciousness returned, and with it memory. Alex realized he was still in danger of falling. He reached with his free right arm and grasped the lower railing. His left arm was around a gate stanchion. It took an effort of will to relax his grip, reach for Kerry’s hand. With her help, he had enough strength to heave his body forward.

Alex lay still, breathing heavily -- but Kerry would not let him relax. She urged him forward, first crawling, then rising to his feet to stagger ahead, leaning on her. The helijet waited just across the wooden bridge, silent in the darkness. She got Alex buckled into the co-pilot's seat. Hot-wiring such standard military transportation had been part of their extensive training. She thanked all the instructors who had forced such dull details on her as she quickly found the inner lights, the tool kit in the left side panel, the right wires . . . no mistakes now, for once the engine started turning the sound would bring guards swarming from below in seconds.

But the training had been effective. The turbojet engine roared to life under her skilled touch. After several long seconds of warm-up, while skipping all but a minimum of the standard pre-flight checks, Kerry gently fed power to the main rotor and lifted off. The helijet was already a distant target in the deceptive moonlight when the first soldiers ran onto the roof and started shooting.

Kerry flew directly out to sea, keeping low. A U.S. fleet constantly prowled in international waters off this unfriendly Pacific coast. Within minutes she had identified herself, was guided to an easy landing on a broad carrier deck, and was shaking hands with a U.S. Navy Captain, brought from the late card game their surprise arrival had interrupted. And then she and Alex were in the sick bay, the rough bandage around her arm was being unwrapped, and another medic was cleaning the blood off the split in Alex's scalp. Then the doctor was saying soothingly that she could sleep now, the arm had to be opened to remove bone chips, and she could fade away into the blessed relief of sleep, deep, healing sleep . . .

Alex and Kerry sat in stiff-backed chairs in the Project Director’s office, trying to look at ease. It had been four days since their escape. A military jump-jet had returned them to the Arizona base next day, and they had been confined in the hospital there since.

Dr. Capshaw came hurrying into his office, a small, wrinkle-faced old man, bald except for a fringe of white hair at ear level. He sat down across the shiny expanse of glass-topped desk and rubbed his eyes wearily, though it was only nine in the morning. This was a man who lived with the daily burden of knowing his work might greatly extend the lifespan of the younger staff members -- but not likely in time to save himself.

“Fortunately for you two, we were able to keep this little escapade out of the media,” Dr. Capshaw began abruptly, in his dry, husky voice. “I trust your little experience at the hands of General Hernandez has convinced you, finally, that our warnings were well-founded?”

Kerry nodded, and Alex muttered agreement.

Dr. Capshaw leaned back in his chair and peered at them over his clasped fingers. “If all goes as expected, we will have seven more couples at your stage within the next year,” the creaky old voice went on. “Until then, I must have your promise that you will not plan to leave the base again. You are both too intelligent not to understand how important your bodies are, young and inexperienced though your minds may be.”

Alex felt his face flush. “I thought we handled ourselves fairly well,” he said, trying to keep his voice level. “We got away by our own efforts. I finally overcame my one real psychological problem, acrophobia. And Kerry is pregnant, which means you should finally have that perfect set of genes you’ve wanted for so long --” He stopped when he saw the twisted grimace that passed for a smile on Dr. Capshaw’s deeply lined face.

Alex glanced at Kerry. She was faintly smiling.

“Sorry, Alex,” said Capshaw. “We still have a long way to go. In the separate debriefings the Intelligence boys put you through, we learned that Kerry spent two days alone in a cell while General Hemandez was assembling his medical specialists. During the second one, she started her period."

Alex caught himself in time to prevent his jaw from dropping. He looked at Kerry. She refused to meet his gaze, but the smile grew wider.

“That's correct,” the doctor said as he came around the desk to usher them out. “Kerry did the only thing she could do that would force you to climb that wall -- she lied like a poker player!”