"The sensors have you," Stone said. His voice was dull.

Stone was sitting in the Level V laboratory, watching on the consoles as the computer electric eyes picked up Hall and outlined his body moving up the wall. To Stone he seemed painfully vulnerable. Stone glanced over at a third screen, which showed the ligamine ejectors pivoting on their wall brackets, the slim barrels coming around to take aim.

"Go!"

On the screen, Hall's body was outlined in red on a vivid green background. As Stone watched, a crosshair was superimposed over the body, centering on the neck. The computer was programmed to choose a region of high blood flow; for most animals, the neck was better than the back.

Hall, climbing up the core wall, was aware only of the distance and his fatigue. He felt strangely and totally exhausted, as if he had been climbing for hours. Then he realized that the gas was beginning to affect him.

"The sensors have picked you up," Stone said. "But you have only ten more yards."

Hall glanced back and saw one of the sensor units. It was aimed directly at him. As he watched, it fired, a small puff of bluish smoke spurting from the barrel. There was a whistling sound, and then something struck the wall next to him, and fell to the ground.

"Missed that time. Keep going."

Another dart slammed into the wall near his neck. He tried to hurry, tried to move faster. Above, he could see the door with the plain white markings LEVEL IV. Stone was right; less than ten yards to go.

A third dart, and then a fourth. He still was untouched. For an ironic moment he felt irritation: the damned computers weren't worth anything, they couldn't even hit a simple target . . .

The next dart caught him in the shoulder, stinging as it entered his flesh, and then there was a second wave of burning pain as the liquid was injected. Hall swore.

Stone watched it all on the monitor. The screen blandly recorded strike and then proceeded to rerun a tape of the sequence, showing the dart moving through the air, and hitting Hall's shoulder. It showed it three times in succession.

The voice said, "There are now two minutes to self-destruct.

"It's a low dose," Stone said to Hall. "Keep going."

Hall continued to climb. He felt sluggish, like a four-hundred pound man, but he continued to climb. He reached the next door just as a dart slammed into the wall near his cheekbone.

"Nasty."

"Go! Go!"

The door had a seal and handle. He tugged at the handle while still another dart struck the wall.

"That's it, that's it, you're going to make it," Stone said.

"There are now ninety seconds to self-destruct," the voice said.

The handle spun. With a hiss of air the door came open. He moved into an inner chamber just as a dart struck his leg with a brief, searing wave of heat. And suddenly, instantly, he was a thousand pounds heavier. He moved in slow motion as he reached for the door and pulled it shut behind him.