There was a sudden, subtle shift in the atmosphere. It was nothing he could explain, yet it was there. The sun was unchanged, undiminished, cresting far to the west of them. The brilliant haze lay across rocks and outcroppings, but there was a difference between this plateau and the land below them.

Troubled, Solo was aware of a faint, but persistent ache in his temples. A headache! Hadn't this been the sign Pete and Marty both noticed first up here?

Something else nagged at Solo. Then he remembered. Mabel had said it. There were no birds, no animals, not even a lizard or a mouse.

He was aware that Mabel had shifted in her saddle and stared back at him, a faint smile twisting her lovely mouth. "What's wrong, Solo?"

He shrugged. "I don't know. I only know that something is wrong."

"It's your imagination."

"Perhaps." Solo reached up, messaging his temples. "Why don't we stop for coffee?"

Mabel laughed, but agreed. They swung down, ground-tied their horses.

Mabel sat on a small boulder. She watched Solo gather grease-wood sticks and start a small fire between two smooth stones. He placed the smoked coffee pot on it; soon the aroma of coffee obscured everything else.

Solo hunkered beside the fire. His eyes ached now, but he remained alert, watchful. He was troubled, though there were no sounds except the crackle of the fire, the bubble of the boiling coffee water, the snuffling of the tethered horses.

"You're scared, Solo." Mabel's voice raked at him.

He glanced up. "Sometimes you have to be smart enough to be scared. Did you know that's how man learned to exist in this world—by being scared first?"

"What scares you up here?" she inquired.

He shook his head. "Everything. Nothing. I've the unshakable feeling that we're being watched."

"Watched?" She laughed. "By whom? By what?"

"I don't know." Solo stared into the fire. "Mabel, something is wrong—and has been for the past hour or so."

Mabel laughed, watching him pour steaming black coffee into tin cups. "It's just your nerves."

He shrugged. "Maybe."

She laughed louder. "Do your corns ache when it rains, Solo?"

He stared at her, frowning. From his vest pocket he removed a small aspirin-sized tin box. He opened it, took out two small purple capsules.

"What's that, Solo?"

He offered her one of the capsules. "It's an antidotes for nerve gas."

"Nerve gas?"

"We may walk into it at any minute, Mabel. Maybe we have already."

She shook her head.

He shrugged, said nothing. She refused to take the capsule. He closed his fist, holding it.

She watched him take a purple capsule, wash it down with the coffee.

"It isn't that I'm not grateful," she said, "but I don't believe we're going to find anything like that up here."

"I hope you're right." Suddenly Solo stiffened.

Mabel stared at him. "What's the matter now?"

Solo came upward slightly, staring past her. "Didn't you hear that?"

She jerked her head around. "I didn't hear anything."

"There it is again," Solo said.

While Mabel was turned, staring across her shoulder, he reached out, opened his fist and dropped the purple capsule into her tin coffee cup.

She turned back, frowning. "You're cracking up, Solo. I didn't hear anything."

Solo sighed and shrugged. He sat back, relaxed, watching her drink down her coffee.

FOUR

"How much further are we climbing before we make camp?" Mabel asked an hour later as they rode falteringly upward.

Solo check the sun.

"Not much longer," he said. "No sense taking a chance riding. A horse can break a leg."

"You worry like a mother hen," Mabel taunted. She prodded her horse, riding suddenly swiftly ahead.

She screamed, throwing her arms up before her face. She twisted, falling from the saddle.

Solo urged his mount forward, but it was as if he rode into an invisible wall. Something struck him and he was driven from his saddle.

Solo went sprawling outward, face first. It was not as though he fell, rather as if he were being thrust downward with terrible force by unseen hands.

The two horses reared, squealing. They tried to run forward, but their way was blocked by this invisible wall. But when they wheeled about, in panic, they were unable to run downhill, either.

Solo struck the ground hard. He felt the savage bite of lava spikes. He rolled along the shale shelf, trying to set himself. He was helpless.

He turned, seeing Mabel huddled on rock outcroppings.

"Mabel!"

He yelled her name again, but she did not answer. She didn't move. He lifted himself slowly to his hands and knees, feeling as if he were fighting incredible downward thrust. He fought against this pressure, lunging upward.

He cried out in agony.

It was as if his head struck solid stone. He shuddered, staggering to his knees, rolled helplessly over upon his back.

For one more moment the mountain side skidded around him, the boulders and the clouds changing places, like skittering bats.

He fought against the darkness that blacked out everything. He pushed upward, but could not rise. But this time when he fell, he went plunging downward into darkness where he was conscious of nothing, not even the pain.

Solo had no idea how long he was unconscious.

He forced his eyes open, conscious of the lancing pain, the throbbing in his temples. It was deep dusk, almost full dark, or else an impairment of vision laid an occluding fog on everything.

He tilted his head, saw that Mabel had not stirred. The horses had fallen, and they lay still on the rocks.

He moved his eyes, searching. Nothing appeared to have altered. The incredible emptiness reached outward in every direction. Ghost Riders, he thought. He tried to drive the mindless idea from his brain. He could not do it. He was convinced that he was surrounded by menacing beings, yet he could not see them. They threw him on the ground, and they held him helplessly when he attempted to rise.

He struggled again to get to his knees, but though there were no ties on him, no ropes, or chains, it was as if he were bound.

The nerve gas.

Stunned, Solo lay helplessly on his back, staring at the darkening sky. He and Mabel had ridden into an invisible wall—odorless, colorless nerve gas, clouds and banks of it. Both Pete and Marty must have ridden up the mountains to this place. This gas was what the two cowpokes had inhaled—the fatal fumes.

It had left them confused, dazed. In the case of Marty, victim of hallucinations—he had died believing he spent three days on a prolonged drunk in the bar at Cripple Bend.

Solo struggled against the invisible bonds immobilizing him.

He stared, eyes wide, trying to find some clouding of that gas. There was nothing visible, but it was there.

If those two cowpunchers had ridden into this bank of nerve gas it had to be piped from some underground storage tanks. And these had to be somewhere nearby—a cave, a well, an abandoned shaft. Something! The answer was that simple, if only he could find it.

Sweating, Solo fought to push himself upward. If he uncovered the cave or shaft from which the gas emanated, he'd have taken a giant step toward answering the riddle of those missing cattle, perhaps a step toward finding those vanished trains and Illya Kuryakin.

He lay, sweating, and his mind raced, though his body was immobile.

Hallucination.

This was the answer. He saw clearly now how this nerve gas had made it possible to move one thousand head of cattle as if they vanished without leaving a trace. No traces would be seen by men who were brainwashed.