She dropped the canister of Vexxon into that natural run, and it slipped down halfway before halting.

She took one of the Uzis from Chris, turned to the approaching car, which was now about seventy-five yards away, and opened fire. She saw bullets punch at least two holes in the windshield. The rest of the tempered glass instantly crazed.

The car-she could see now that it was a Toyota-spun out, turning a full three hundred and sixty degrees, then ninety degrees more, throwing up clouds of dust, tearing through a couple of still green tumbleweeds. It came to rest about forty yards from the Buick, sixty yards from her and Chris, the front end pointed north. Doors flew open on the far side. Laura knew the occupants were scrambling out of the car where she would not see them, staying low.

She took the other Uzi from Chris and said, “Into the slide, kiddo. When you reach the canister of gas, push it ahead of you all the way to the bottom.”

He went down the wall of the arroyo, pulled most of the way by the force of gravity but having to scoot along a couple of times when friction stopped him. It was exactly the kind of daredevil stunt that would have raised a mother's ire under other circumstances, but now she cheered him on.

She pumped at least a hundred rounds into the Toyota, hoping to pierce the fuel tank and set off the gasoline with a bullet-made spark, roasting the bastards as they huddled against the far side. But she emptied the magazine without the desired result.

When she stopped shooting, they took a crack at her. She did not stay long enough to give them a target. With the second Uzi held before her in both hands, she sat on the edge of the arroyo and shoved off into the slide that Chris had already used. In seconds she was at the bottom.

Dry tumbleweeds had blown down to the floor of the gulch from the desert above. Gnarled driftwood, some time-grayed lumber washed from the distant ruins of an old desert shack, and a few stones littered the powder-soft soil that formed the bed of the arroyo. None of those things offered a place to hide or protection from the gunfire that would soon be directed down at them.

“Mom?” Chris said, Meaning: What now?

The arroyo would have scores of tributaries spread out across the desert, and many of those tributaries would have tributaries of their own. The drainage network was like a maze. They could not hide in it forever, but perhaps by putting a few branches of the system between themselves and their pursuers, they would gain time to plan an ambush.

She said, “Run, baby. Follow the main arroyo, take the first right-hand branch you come to, and wait there for me.”

“What're you going to do?”

“I'll wait for them to look over the edge up there,” she said, pointing to the top of the palisades, “then pick them off if I can. Now go, go.”

He ran.

Leaving the canister of Vexxon in plain sight, Laura returned to the wall of the arroyo down which they'd slid. She went to a different vertical channel, however, one that was carved deeper into the wall, had less of a slope, and was half-blocked at its midpoint by a mesquite bush. She stood in the bottom of that deep hollow, confident that the bush overhead blocked their view of her from the desert above.

To the east, Chris vanished around a turn into a tributary of the main channel.

A moment later she heard voices. She waited, waited, giving them time to feel confident that both she and Chris were gone. Then she stepped out from the erosion channel in the arroyo wall, turned, and swept the top of the cliff with bullets.

Four men were there, peering down, and she killed the first two, but the third and fourth leaped backward, out of sight before the arc of fire reached them. One of the bodies lay at the top of the arroyo wall, one arm and leg over the brink. The other fell all the way to the floor of the channel, losing his sunglasses on the way.

March 16, 1944. The institute.

When the bottle with the message did not bounce back to him, Stefan was reasonably confident that it had reached Laura before she had been killed, only seconds after he had first departed for 1944.

Now he returned to the programmer's desk and set to work on the calculations that would return him to the desert a few minutes after his previous arrival there. He could make that trip because he would be arriving subsequent to his previous hasty departure, and there would be no possibility of encountering himself, no paradox.

Again the calculations were not terribly difficult because he needed only to work forward from the numbers that the IBM PC had provided him. Though he knew that the time he spent here was not passing in equal measure in the desert of 1989, he was eager to rejoin Laura nevertheless. Even if she had taken the advice of the message in the bottle, even if the future he had seen had been changed and she was still alive, she would have to deal with those SS gunmen, and she would need help.

In forty minutes he had the numbers that he required, and he reprogrammed the gate.

Again he opened the panel on the jaunt recorder and tore the evidence off that spool of paper.

Carrying the Uzi and the pistol, gritting his teeth as the dull throbbing in his half-healed shoulder grew worse, he entered the gate.

Lugging both the Vexxon canister and the Uzi, Laura joined Chris in the narrower tributary off the main channel, about sixty feet from the point at which they had descended into the system. Crouching at the corner formed by the two earth walls, she looked back into the primary arroyo from which she had come.

On the desert above, one of the surviving assassins shoved the dangling corpse off the brink, into the deep gulch, apparently to see if she was still immediately below them and if she would be tricked into opening fire. When there was no fire, the two survivors became bolder. One lay at the brink with a submachine gun, covering the other man while he slid down. Then the first gunman covered the second's descent.

When the second man joined the first, Laura stepped boldly around the corner and squeezed off a two-second burst. Both of her pursuers were so startled by her aggressiveness that they did not return fire but threw themselves toward the deep, vertical erosion channels in the arroyo wall, seeking shelter there as she had sheltered while waiting for the opportunity to shoot them off the top of the cliff. Only one of them made it to cover. She blew the other one away.

She stepped back around the corner, picked up the cylinder of nerve gas, and said to Chris, “Come on. Let's hustle.”

As they ran along the tributary, seeking yet another branch in the maze, lightning and thunder split the blue sky above.

“Mr. Krieger!” Chris said.

He returned to the desert seven minutes after he had originally departed for his meetings with Churchill and Hitler in 1944, just two minutes after his initial return when he had seen Laura and Chris dead at the hands of SS gunmen. There were no bodies this time, just the Buick-and the bullet-riddled Toyota in a different position.

Daring to hope that his scheme had worked, Stefan hurried to the arroyo and ran along the brink, searching for someone, anyone, friend or foe. Before long he saw the three dead men on the floor of the channel, thirty feet below.

There would be a fourth. No SS squad would have been composed of only three men. Somewhere in the network of zigzagging arroyos that crossed the desert like a chain of jagged lightning bolts, Laura was still on the run from the last man.

In the arroyo wall Stefan found a vertical channel that appeared to have been used already; he stripped off his book-filled rucksack, slid to the bottom. On the way down, his back scraped against the earth, and hot pain flared in the partly healed exit wound. At the end of the slope, when he stood up, a wave of dizziness washed through him, and bile rose in his throat.

Somewhere in the maze to the east, automatic weapons chattered.

She halted just inside the mouth of a new tributary and signaled Chris to be quiet.

Breathing through her open mouth, she waited for the last killer to turn the corner into the channel that she had just left. Even in the soft soil, his running footsteps were audible.

She leaned out to gun him down. But he was extremely cautious now; he entered low and at a dead run. When her gunfire alerted him to her position, he crossed the channel and hid against the same wall off which her new tributary opened, so she could get a clear shot at him only if she stepped out into the arroyo where he waited. In fact she tried that, risking his fire, but when she squeezed off a two-second burst, it ended in less than a second. The Uzi spat out its last ten or twelve rounds, then failed her.

Klietmann heard her Uzi go empty. He looked out from the crevice in the arroyo wall where he was sheltering and saw her throw the gun down. She disappeared into the mouth of the tributary where she had been laying for him.

He considered what he had seen in the Buick, up on the desert: a .38 revolver lying on the driver's seat. He assumed that she had not had time to grab it or, in her haste to get that curious canister from the trunk, had forgotten about the handgun.

She'd had two Uzis, both discarded now. Could she have had two handguns-and left only one in the car?

He thought not. Two automatic carbines made sense because they were useful at a distance and in a variety of circumstances. But unless she was an expert marksman, a handgun would be of little use except at close range, where six shots was about all she would need before she either dealt with her assailant or died at his hands. A second revolver would be superfluous.

Which meant that for self-defense she had-what? That canister? It had looked like nothing more than a chemical fire extinguisher.

He went after her.

The new tributary was narrower than the one before it, just as that one had been narrower than the main channel. It was twenty-five feet deep and only ten feet wide at the mouth, growing shallower and half that narrow as it cut a crooked path through the desert floor. In a hundred yards, it funneled to an end.

At the terminus, she looked for a way out. On two sides the cliffs were too steep, soft, and crumbly to be easily climbed, but the wall behind her sloped at a scalable angle and was studded with mesquite that offered handholds. She knew, however, that they would be only halfway up the slope when their pursuer found them; suspended on that high ground, they would make easy targets.

This was where she would have to make her last stand.

Cornered at the bottom of this big, natural ditch, she looked up at the rectangular patch of blue sky and thought they might have been at the bottom of an enormous grave in a cemetery where only giants were buried.

Destiny struggles to reassert the pattern that was meant to be.

She pushed Chris behind her, into the point of the dead-end arroyo. Ahead of her, she could see forty feet back the way they had come, along the five-foot-wide channel, to the point where it angled to the left. He would appear at that turn within a minute or two.

She dropped to her knees with the canister of Vexxon, intending to strip the safety wire off the manual trigger. But the wire was not merely looped and braided through the trigger; it was repeatedly wound and then sealed with solder. It could not be unwound; it had to be cut, and she had nothing with which to cut it.

Maybe a stone. A sharp-edged stone might wear through the wire if scraped across it often enough.

“Get me a stone,” she said urgently to the boy behind her. “One with a rough, sharp edge.”

As he searched the soft, flood-carried soil that had washed down from the desert floor, looking for a suitable scrap of slate, she examined the automatic timer on the canister, which provided a second means of releasing the gas. It was a simple device: a rotating dial was calibrated in minutes; if you wanted to set the timer for twenty minutes, you twisted the dial until the 20 was lined up with the red mark on the dial frame; when you pushed the button in the center, the countdown began.

The problem was that the dial could be set for no fewer than five minutes. The gunman would reach them sooner than that.

Nevertheless she twisted the dial to 5 and pushed the button that started it ticking.

“Here, Mom,” Chris said, presenting her with a blade of slate that just might do the job.

Though the timer was ticking, she set to work, frantically sawing at the strong, twined wire that prevented manual release. Every few seconds she looked up to see if the assassin had found them, but the narrow arroyo ahead of them remained deserted.

Stefan followed the footprints in the soft soil that formed the bed of the arroyo. He had no idea how far behind them he might be. They had only a few minutes' head start, but they were probably moving faster than he was because the pain in his shoulder, exhaustion, and dizziness slowed him.

He had unscrewed the silencer from the pistol, thrown it away, and tucked the handgun under his belt. He carried the Uzi in both hands, at the ready.

Klietmann had thrown away his Ray-Bans because the floor of the arroyo network was shadow-swaddled in many places, especially as they moved into narrower tributaries, where the walls closed in and left less of an opening above for sunlight to enter.

His Bally loafers filled with sand and provided no surer footing here than on the slate of the desert above. Finally he paused, kicked off the shoes, stripped off the socks, and proceeded barefoot, which was a great improvement.

He was not tracking the woman and the boy as swiftly as he would have liked, partly because of the shoes that he had discarded, but mainly because he kept a watch on his backside every step of the way. He had heard and seen the recent display of thunder and lightning; he knew Krieger must have returned. Most likely, as Klietmann stalked the woman and boy, Krieger was stalking him. He did not intend to be meat for that tiger.

On the timer two minutes had ticked off.

Laura had sawed almost as long at the wire, initially with the blade of slate that Chris had found, then with a second that he turned up when the first piece crumbled in her fingers. The government could not make a postage stamp that could be trusted to stay on an envelope, could not build a battle tank that was capable of crossing a river on every attempt, could not protect the environment or eliminate poverty, but it sure as hell knew how to procure indestructible wire; this stuff must be some wonder material which they had developed for the space shuttle and for which they'd eventually found more mundane uses; it was the wire God would use to guy the tilting pillars that held up the world.

Her fingers were raw, the second chip of slate was slick with her blood, and only half the strands of wire were cut when the barefoot man in black slacks and a white shirt rounded the bend in the narrow arroyo, forty feet away.

Klietmann edged forward warily, wondering why the hell she was struggling so frantically with the fire extinguisher. Did she ' really think a blast of chemical fog would disorient him and protect her from submachine-gun fire?

Or was the extinguisher not what it appeared to be? Since arriving in Palm Springs less than two hours ago, he had encountered several things that were not what they appeared to be. A red curb, for instance, did not mean EMERGENCY PARKING, as he had thought, but NO PARKING AT ANY TIME. Who could know? And who could know for sure about this canister with which she was struggling?