I could hardly move, but I jerked and screamed and writhed against the tapes, making as much noise as I possibly could. Lyle laughed delightedly. Between us we were making a frightful din. How much of my screaming was genuine panic? I don’t know, but I’m sure a good deal of it was, because my chair was rolling steadily closer to the edge without being pushed. There was a small lip at the very brink of the pit, but it might not be enough to halt the forward motion.

I was an arm’s length from death — the chair nearing the edge — Van Lyle walking by my side and peering at my face, savoring my expression.

Then, from behind — at last — came the squeak of unoiled wheels.

Before Van Lyle could move, McAndrew was on him. Mac had freed his legs and was out of the wheelchair. Smarter than I would have been, he came forward in one silent rush, pushing the wheelchair in front of him like a ram. The edge of the seat caught Lyle behind the thighs at knee level. He fell backward into a sitting position. Before he could cry out he was at the edge. He and the chair went right on over. There was a scream and a great splash. McAndrew halted at the brink, staring down.

“Mac!” I screamed. I was still rolling.

He half-turned and threw himself in front of my wheelchair, stopping it with his own body. At the very edge, we both peered into the pit.

Lyle had gone in flat and facedown. He rose to the surface in a cloud of steam, screaming and clawing at his eyes. As we watched, his hair and skin began to smoke and frizzle. His arms waved and thrashed down on either side of him. Then he went under again.

The vat was more corrosive than Lyle had suggested. Twice more he rose, howling in agony. But the liquid must have reached his lungs. By the time he went under for the last time he was silent, a dark-green mass that was already losing its human form.

And while Van Lyle was dying, McAndrew kept tearing at the tapes that held me. I think it was the only thing that kept him from plunging in himself to try to help.

The tapes were strong, and Mac’s hands were trembling. It was two more minutes before I could stand up, advance shakily to the brink, and peer down into the choking green fumes. I saw a dark vat, with sluggish ripples moving across the surface. About ten feet away from the edge floated an amorphous rounded lump.

“Don’t look,” McAndrew said. “He’s dead.”

“Of course he is. But it was his own damned fault.”

I don’t know how angry I sounded, but Mac winced. “Come on, Jeanie,” he said. “It’s over now. Let’s get out of here.”

“It’s not.” And then, when he stared at me. “It’s not over. Not yet. Come on, Mac. I may need help.”

I ran back, through the sequence of chambers that threaded the food production facility like beads on a necklace. Anna Griss sat at a table in the third one, calmly reading. She had just enough time to cry out in surprise before I reached her.

I lashed out and caught her with my fist high on the left cheek. While she was still reeling backward, partly stunned, I grabbed her in a neck lock.

“Come on, Mac. Help me. Back to the vats.”

He wasn’t much use, but it didn’t matter. My own adrenaline level was so high, I could easily have carried her all the way myself. She was faintly struggling when I thrust her into the remaining wheelchair. The sticky tape that had held me was no good any more, but the cords that had bound Mac were enough to tie her.

I wheeled her to the very edge, so that the acrid corrosive vapors filled her throat and mine.

“That’s Van Lyle down there.” I pointed to the sodden green hulk, floating almost submerged. “You’re going after him.”

“Ohmygod. No, no.” She was panting, shaking her head with its newly disordered hair and smudged make-up. “Don’t push me over. Don’t kill me. Please.”

“You were ready enough to see us killed. Here you go, Anna Griss.” I tilted the chair far forward, so that all that held her from the vat were her bonds. “This is what people get who mess around with me. You’re dead.”

I put my face close to hers. She was too frightened for tears, but her staring eyes were watering in the poisonous fumes.

“Say your prayers,” I whispered. “Say goodbye.”

“No. Oh, please.” She was straining back, away from the deadly vat. “Don’t. I’ll do anything. Anything!”

“Jeanie!” cried McAndrew.

“Shut up, Mac. This is between me and her.”

I moved the chair back a couple of feet and walked around in front of it to gaze into her eyes. “Look at me, Anna Griss. I’m not going to kill you — this time. But one more problem with you and you’re dead. Do you understand? If the sight or sound or smell of you crosses my path again, ever, I’ll come after you. And I’ll get you. Don’t ever doubt that. I’ll get you.”

She did not speak, but she nodded. I turned to McAndrew.

“I think she has the message. If she annoys either of us again, she’ll be pig feed. Come on, Mac.”

“You can’t just leave her! She might go over the edge.”

“If she goes, she goes.” I grabbed his arm. “No big loss. But you and I are leaving. Come on.”

He kept turning to look at her, but he allowed me to lead him away. I did not look back.

“You wouldn’t have, would you?” he said at last, when we had walked through half a dozen chambers. “You wouldn’t have killed her, no matter what she did.”

It was obvious what he wanted to hear. “No. I wouldn’t have killed her.”

“Then why did you do all that, threatening her?”

“Because I had to. I might as well ask, why did you push Van Lyle over.”

“But he was going to kill you! I didn’t think — I just did it. Like you when you were screaming, going toward the pit. You just did it. What a bit of luck that was, Lyle leaving your mouth untaped! Otherwise you’d not have been able to scream, and when I ran at him he would have heard me coming.”

I don’t usually care what credit I get. But that was a bit too much.

“Mac,” I said. “Listen to me. I’m going to tell you something about the invariants of nature. You have yours for physics and mathematics, determinants and momentum and conserved vector currents. And I have mine — the invariants of human nature: Love, and jealousy, and fear, and hate. Van Lyle was a cruel, sadistic bastard. He was like that when we first met him, he was like that out in the Oort cloud, and he was still like that until the moment he died. He couldn’t change his nature. I deliberately told him that I wouldn’t be able to beg and scream and grovel with my mouth taped. After that there was absolutely no way he’d muzzle me — no matter what Anna Griss told him to do. He wanted to see my terror, and hear my screams. And so I had the chance to free you.”

McAndrew is an innocent soul. He was shocked silent by what I said. Finally he sighed, and muttered, “Maybe you’re right. But I don’t see why you did that to Anna Griss.”

“I had to — because in her own way, she’s no different from him. She has her own invariants: power, and control, and fear. Anna won’t hold back on revenge to be nice to anyone. She’ll go on, as far as she can go, until she’s stopped. You and I have just stopped her. But we could never have done that by persuasion, or logic. She had to look death in the face for herself, and stare right down his black gullet.”

“She could still cause us trouble. She could come after us, on Earth or off it.”

“She could, but she won’t. She’d like to get us, but she’ll remember that pit. Anna understands power. If her people tried and failed again, she knows I’ll come after her. The pleasure of finishing me isn’t worth the risk.”

“Are you sure?”

“Yes, I am. Mac, trust me. If I don’t question your spinors and twistors and calibration of optical scalars, you shouldn’t second guess me on Anna Griss.”

“So you think it’s safe to go back to the Geotron, and see how the experiments came out? I left before the results were in, you see, because of what you said to me.”