I glanced at the path we had made across the Mare Serenitatis. The Latin translated as "the Sea of Serenity." It was well named because, as far as the eye could see in every direction, there was a smooth layer of pumice that resembled the surface of a calm sea. Scattered across the quiet sea of virgin Moon dust were occasional islands of rock that jutted abruptly toward the infinity of stars above. Considering everything, our surroundings conveyed a sense of serenity like none I had ever felt.

Our bounding path across the level expanse was clearly marked. Because of the light gravity, we had leaped high into the air with each step and every time we struck the ground, the impact had raised a cloud of dustlike pumice. Now the clouds of dust were slowly settling in the light gravity.

Above us, the stars were cold, motionless and crystal-clear. Indifferently, they sprayed a faint light on our surroundings ... a dim glow that was hardly sufficient for normal vision and was too weak to be reflected toward Earth.

We turned our head-lamps on the strange object before us. Five beams of light illuminated the smooth shape that protruded from the Moon's surface.

The incongruity was so awesome that for several minutes, we remained motionless and quiet. Miller broke the silence with his quavering voice, "Strange someone didn't notice it before."

Strange? The object rose a quarter of a mile above us, a huge, curving hulk of smooth metal. It was featureless and yet conveyed a sense of alienness. It was alien and yet it wasn't a natural formation. Something had made the thing, whatever it was. But was it strange that it hadn't been noticed before? Men had lived on the Moon for over a year, but the Moon was vast and the Mare Serenitatis covered three hundred and forty thousand square miles.

"What is it?" Marie asked breathlessly.

Her husband grunted his bafflement. "Who knows? But see how it curves? If it's a perfect sphere, it must be at least two miles in diameter!"

"If it's a perfect sphere," Miller suggested, "most of it must be beneath the Moon's surface."

"Maybe it isn't a sphere," my wife said. "Maybe this is all of it."

"Let's call Lunar City and tell the authorities about it." I reached for the radio controls on my suit.

Kane grabbed my arm. "No. Let's find out whatever we can by ourselves. If we tell the authorities, they'll order us to leave it alone. If we discover something really important, we'll be famous!"

I lowered my arm. His outburst seemed faintly childish to me. And yet it carried a good measure of common sense. If we discovered proof of an alien race, we would indeed be famous. The more we discovered for ourselves, the more famous we'd be. Fame was practically a synonym for prestige and wealth.

"All right," I conceded.

Miller stepped forward, moving slowly in the bulk of his spacesuit. Deliberately, he removed a small torch from his side and pressed the brilliant flame against the metal.

A few minutes later, the elderly mineralogist gave his opinion: "It's steel ... made thousands of years ago."

Someone gasped over the intercom, "Thousands of years! But wouldn't it be in worse shape than this if it was that old?"

Miller pointed at the small cut his torch had made in the metal. The notch was only a quarter of an inch deep. "I say steel because it's similar to steel. Actually, it's a much stronger alloy. Besides that, on the Moon, there's been no water or atmosphere to rust it. Not even a wind to disturb its surface. It's at least several thousand years old."

We slowly circled the alien structure. Several minutes later, Kane shouted, "Look!"

A few feet above the ground, the structure's smooth surface was broken by a circular opening that yawned invitingly. Kane ran ahead and flashed his head-lamp into the dark recess.

"There's a small room inside," he told us, and climbed through the opening.

We waited outside and focused our lamps through the five-foot opening to give him as much light as possible.

"Come on in, Marie," he called to his wife. "This is really something! It must be an alien race. There's all kinds of weird drawings on the walls and gadgets that look like controls for something...."

Briefly, my lamp flickered over Marie's pale face. Her features struggled with two conflicting emotions: She was frightened by the alienness of the thing and yet she wanted to be with her husband. She hesitated momentarily, then climbed through the passage.

"You want to go in?" my wife asked.

"Do you?"

"Let's."

I helped Verana through the opening, climbed through myself and turned to help Miller.

Miller was sixty years old. He was an excellent mineralogist, alert mentally, but with a body that was almost feeble. I reached out to help him as he stepped into the passageway.

For a brief second, he was framed in the opening, a dark silhouette against the star-studded sky.

The next second, he was thrown twenty yards into the air. He gasped with pain when he struck the ground. "Something pushed me!"

"Are you all right?"

"Yes."

He had fallen on a spot beyond our angle of vision. I started through the passage....

... and struck an invisible solid wall.

My eyes were on the circular opening. A metal panel emerged from a recess on one side and slid across the passage. The room darkened with the absence of starlight.

"What happened?"

"The door to this damned place closed," I explained.

"What?"

Before we could recover from the shock, the room filled with a brilliant glare. We turned off our lamps.

The room was approximately twelve feet long and nine feet wide. The ceiling was only a few inches above our heads and when I looked at the smooth, hard metal, I felt as if I were trapped in some alien vault.

The walls of the room were covered with strange drawings and instruments. Here and there, kaleidoscopic lights pulsed rhythmically.

Kane brushed past me and beat his gloved fists against the metal door that had imprisoned us.

"Miller!"

"Yes?"

"See if you can get this thing open from the outside."

I knelt before the door and explored its surface with my fingers. There were no visible recesses or controls.

Over the intercom network, everyone's breath mingled and formed a rough, harsh sound. I could discern the women's quick, frightened breaths that were almost sobs. Kane's breath was deep and strong; Miller's was faltering and weak.

"Miller, get help!"

"I'll—" The sound of his breathing ceased. We listened intently.

"What happened to him?"

"I'll phone Lunar City." My fingers fumbled at the radio controls and trembled beneath the thick gloves.

I turned the dials that would connect my radio with Lunar City....

Static grated against my ear drums.

Static!

I listened to the harsh, erratic sound and my voice was weak by comparison: "Calling Lunar City."

"Static!" Kane echoed my thoughts. His frown made deep clefts between his eyebrows. "There's no static between inter-lunar radio!"

Verana's voice was small and frightened. "That sounds like the static we hear over the bigger radios when we broadcast to Earth."

"It does," Marie agreed.

"But we wouldn't have that kind of static over our radio, unless—" Verana's eyes widened until the pupils were surrounded by circles of white—"unless we were in outer space!"

We stared at the metal door that had imprisoned us, afraid even to speak of our fantastic suspicion.

I deactivated my radio.

Marie screamed as an inner door opened to disclose a long, narrow corridor beyond.

Simultaneous with the opening of the second door, I felt air press against my spacesuit. Before, our suits had been puffed outward by the pressure of air inside. Now our spacesuits were slack and dangling on our bodies.

We looked at each other and then at the inviting corridor beyond the open door.

We went single file, first Kane, then his wife Marie. Verana followed next and I was the last.

We walked slowly, examining the strange construction. The walls were featureless but still seemed alien. At various places on the walls were the outlines of doors without handles or locks.

Kane pressed his shoulder against a door and shoved. The door was unyielding.

I manipulated the air-vent controls of my spacesuit, allowed a small amount of the corridor's air into my helmet and inhaled cautiously. It smelled all right. I waited and nothing happened. Gradually, I increased the intake, turned off the oxygenating machines and removed my helmet.

"Shut off your oxy," I suggested. "We might as well breathe the air in this place and save our supply. We may need the oxygen in our suits later."

They saw that I had removed my helmet and was still alive and one by one removed their own helmets.

At the end of the corridor, Kane stopped before a blank wall. The sweat on his face glistened dully; his chest rose and fell rapidly. Kane was a pilot and one of the prerequisites for the job of guiding tons of metal between Earth and the Moon was a good set of nerves. Kane excited easily, his temper was fiery, but his nerves were like steel.

"The end of the line," he grunted.

As though to disprove the statement, a door on his right side opened soundlessly.

He went through the doorway as if shoved violently by an invisible hand.

The door closed behind him.

Marie threw herself at the door and beat at the metal. "Harry!"

Verana rushed to her side. Another door on the opposite side of the corridor opened silently. The door was behind them; they didn't notice.

Before I could warn them, Marie floated across the corridor, through the doorway.

Verana and I stared at the darkness beyond the opening, our muscles frozen by shock.

The door closed behind Marie's screaming, struggling form.

Verana's face was white with fear. Apprehensively, she glanced at the other doors that lined the hall.

I put my arms around her, held her close.

"Antigravity machines, force rays," I suggested worriedly.

For several minutes, we remained motionless and silent. I recalled the preceding events of the day, searched for a sense of normality in them. The Kanes, Miller, Verana and I lived in Lunar City with hundreds of other people. Mankind had inhabited the Moon for over a year. Means of recreation were scarce. Many people explored the place to amuse themselves. After supper, we had decided to take a walk. As simple as that: a walk on the Moon.

We had expected only the familiar craters, chasms and weird rock formations. A twist of fate and here we were: imprisoned in an alien ship.

My legs quivered with fatigue, my heart throbbed heavily, Verana's perfume dizzied me. No, it wasn't a dream. Despite our incredible situation, there was no sensation of unreality.

I took Verana's hand and led her down the long corridor, retracing our steps.

We had walked not more than two yards when the rest of the doors opened soundlessly.

Verana's hand flew to her mouth to stifle a gasp.

Six doors were now open. The only two that remained closed were the ones that the Kanes had unwillingly entered.

This time, no invisible hand thrust us into any of the rooms.

I entered the nearest one. Verana followed hesitantly.

The walls of the large room were lined with shelves containing thousands of variously colored boxes and bottles. A table and four chairs were located in the center of the green, plasticlike floor. Each chair had no back, only a curving platform with a single supporting column.

"Ed!" I joined Verana on the other side of the room. She pointed a trembling finger at some crude drawings. "The things in this room are food!"

The drawings were so simple that anyone could have understood them. The first drawing portrayed a naked man and woman removing boxes and bottles from the shelves. The second picture showed the couple opening the containers. The third showed the man eating from one of the boxes and the woman drinking from a bottle.

"Let's see how it tastes," I said.

I selected an orange-colored box. The lid dissolved at the touch of my fingers.

The only contents were small cubes of a soft orange substance.

I tasted a small piece.

"Chocolate! Just like chocolate!"

Verana chose a nearby bottle and drank some of the bluish liquid.

"Milk!" she exclaimed.

"Perhaps we'd better look at the other rooms," I told her.

The next room we examined was obviously for recreation. Containers were filled with dozens of strange games and books of instructions in the form of simple drawings. The games were foreign, but designed in such a fashion that they would be interesting to Earthmen.

Two of the rooms were sleeping quarters. The floors were covered with a spongy substance and the lights were dim and soothing.

Another room contained a small bathing pool, running water, waste-disposal units and yellow cakes of soap.

The last room was an observatory. The ceiling and an entire wall were transparent. Outside, the stars shone clearly for a few seconds, then disappeared for an equal time, only to reappear in a different position.

"Hyper-space drive," Verana whispered softly. She was fascinated by the movement of the stars. For years, our scientists had sought a hyperspatial drive to conquer the stars.

We selected a comfortable chair facing the transparent wall, lit cigarettes and waited.

A few minutes later, Marie entered the room.

I noticed with some surprise that her face was calm. If she was excited, her actions didn't betray it.

She sat next to Verana.

"What happened?" my wife asked.

Marie crossed her legs and began in a rambling manner as if discussing a new recipe, "That was really a surprise, wasn't it? I was scared silly, at first. That room was dark and I didn't know what to expect. Something touched my head and I heard a telepathic voice—"

"Telepathic?" Verana interrupted.

"Yes. Well, this voice said not to worry and that it wasn't going to hurt me. It said it only wanted to learn something about us. It was the oddest feeling! All the time, this voice kept talking to me in a nice way and made me feel at ease ... and at the same time, I felt something search my mind and gather information. I could actually feel it search my memories!"

"What memories?" I inquired.

She frowned with concentration. "Memories of high school mostly. It seemed interested in English and history classes. And then it searched for memories of our customs and lives in general...."

Kane stalked into the room at that moment, his face red with anger. "Do you know where we are?" he demanded. "When those damned aliens got me in that room, they explained what this is all about. We're guinea pigs!"

"Did they use telepathy to explain?" Verana asked. I suddenly remembered that she was a member of a club that investigated extra-sensory perception with the hope of learning how it operated. She was probably sorry she hadn't been contacted telepathically.

"Yeah," Kane replied. "I saw all sorts of mental pictures and they explained what they did to us. Those damned aliens want us for their zoo!"

"Start at the beginning," I suggested.

He flashed an angry glance at me, but seemed to calm somewhat. "This ship was made by a race from another galaxy. Thousands of years ago, they came to Earth in their spaceships when men were primitives living in caves. They wanted to know what our civilization would be like when we developed space flight. So they put this ship on the Moon as a sort of booby-trap. They put it there with the idea that when we made spaceships and went to the Moon, sooner or later, we'd find the ship and enter it—like rabbits in a snare!"

"And now the booby-trap is on its way home," I guessed.

"Yeah, this ship is taking us to their planet and they're going to keep us there while they study us."

"How long will the trip take?" I asked.

"Six months. We'll be bottled up in this crate for six whole damned months! And when we get there, we'll be prisoners!"

Marie's hypnotic spell was fading and once more her face showed the terror inside her.

"Don't feel so bad," I told Kane. "It could be worse. It should be interesting to see an alien race. We'll have our wives with us—"

"Maybe they'll dissect us!" Marie gasped.

Verana scoffed. "A race intelligent enough to build a ship like this? A race that was traveling between the stars when we were living in caves? Dissection is primitive. They won't have to dissect us in order to study us. They'll have more advanced methods."

"Maybe we can reach the ship's controls somehow," Kane said excitedly. "We've got to try to change the ship's course and get back to the Moon!"

"It's impossible. Don't waste your time." The voice had no visible source and seemed to fill the room.

Verana snapped her fingers. "So that's why the aliens read Marie's mind! They wanted to learn our language so they could talk to us!"

Kane whirled in a complete circle, glaring at each of the four walls. "Where are you? Who are you?"

"I'm located in a part of the ship you can't reach. I'm a machine."

"Is anyone else aboard besides ourselves?"

"No. I control the ship." Although the voice spoke without stilted phrases, the tone was cold and mechanical.

"What are your—your masters going to do with us?" Marie asked anxiously.

"You won't be harmed. My masters merely wish to question and examine you. Thousands of years ago, they wondered what your race would be like when it developed to the space-flight stage. They left this ship on your Moon only because they were curious. My masters have no animosity toward your race, only compassion and curiosity."

I remembered the way antigravity rays had shoved Miller from the ship and asked the machine, "Why didn't you let our fifth member board the ship?"

"The trip to my makers' planet will take six months. There are food, oxygen and living facilities for four only of your race. I had to prevent the fifth from entering the ship."

"Come on," Kane ordered. "We'll search this ship room by room and we'll find some way to make it take us back to Earth."

"It's useless," the ship warned us.

For five hours, we minutely examined every room. We had no tools to force our way through solid metal walls to the engine or control rooms. The only things in the ship that could be lifted and carried about were the containers of food and alien games. None were sufficiently heavy or hard enough to put even a scratch in the heavy metal.

Six rooms were open to our use. The two rooms in which the Kanes had been imprisoned were locked and there were no controls or locks to work on.

The rooms that we could enter were without doors, except the ones that opened into the corridor.

After intensive searching, we realized there was no way to damage the ship or reach any section other than our allotted space.

We gave up.

The women went to the sleeping compartments to rest and Kane I went to the "kitchen."

At random, we sampled the variously colored boxes and bottles and discussed our predicament.

"Trapped," Kane said angrily. "Trapped in a steel prison." He slammed his fist against the table top. "But there must be a way to get out! Every problem has a solution!"

"You sure?" I asked.

"What?"

"Does every problem have a solution? I don't believe it. Some problems are too great. Take the problem of a murderer in our civilization: John Doe has killed someone and his problem is to escape. Primarily, a murderer's problem is the same principle as ours. A murderer has to outwit an entire civilization. We have to outwit an entire civilization that was hundreds of times more advanced than ours is now when we were clubbing animals and eating the meat raw. Damned few criminals get away these days, even though they've got such crowds to lose themselves in. All we have is a ship that we can't control. I don't think we have a chance."

My resignation annoyed him. Each of us had reacted differently: Kane's wife was frightened, Verana was calm because of an inner serenity that few people have, I was resigned and Kane was angry.

For several minutes, we sampled the different foods. Every one had a distinctive flavor, comparable to that of a fruit or vegetable on Earth.

Kane lifted a brown bottle to his lips, took a huge gulp and almost choked.

"Whiskey!"

"My masters realized your race would develop intoxicants and tried to create a comparable one," the machine explained.

I selected a brown bottle and sampled the liquid. "A little stronger than our own," I informed the machine.

We drank until Kane was staggering about the room, shouting insults at the alien race and the mechanical voice that seemed to be everywhere. He beat his fist against a wall until blood trickled from bruised knuckles.

"Please don't hurt yourself," the machine pleaded.

"Why?" Kane screamed at the ceiling. "Why should you care?"

"My masters will be displeased with me if you arrive in a damaged condition."

Kane banged his head against a bulkhead; an ugly bruise formed rapidly. "Shtop me, then!"

"I can't. My masters created no way for me to restrain or contact you other than use of your language."

It took fully fifteen minutes to drag Kane to his sleeping compartment.

After I left Kane in his wife's care, I went to the adjoining room and stretched out on the soft floor beside Verana.

I tried to think of some solution. We were locked in an alien ship at the start of a six months' journey to a strange planet. We had no tools or weapons.

Solution? I doubted if two dozen geniuses working steadily for years could think of one!

I wondered what the alien race was like. Intelligent, surely: They had foreseen our conquest of space flight when we hadn't even invented the wheel. That thought awed me—somehow they had analyzed our brains thousands of years ago and calculated what our future accomplishments would be.

They had been able to predict our scientific development, but they hadn't been able to tell how our civilization would develop. They were curious, so they had left an enormously elaborate piece of bait on the Moon.

The aliens were incredibly more advanced than ourselves. I couldn't help thinking, And to a rabbit in a snare, mankind must seem impossibly clever.

I decided to ask the machine about its makers in the "morning."

When I awoke, my head was throbbing painfully.

I opened my eyes and blinked several times to make sure they were functioning properly. I wasn't in the compartment where I had fallen asleep a few hours before.

I was tied to one of the chairs in the "kitchen." Beside me, Verana was bound to a chair by strips of cloth from her skirt, and across from us, Marie was secured to another chair.

Kane staggered into the room. Although he was visibly drunk, he appeared more sober than the night before. His dark hair was rumpled and his face was flushed, but his eyes gleamed with a growing alertness.

"Awake, huh?"

"What have you done, Harry?" his wife screamed at him. Her eyes were red with tears and her lips twisted in an expression of shame when she looked at him.

"Obvious, isn't it? While all of you were asleep, I conked each of you on the head, dragged you in here and tied you up." He smiled crookedly. "It's amazing the things a person can do when he's pickled. I'm sorry I had to be so rough, but I have a plan and I knew you wouldn't agree or cooperate with me."

"What's your plan?" I asked.

He grinned wryly and crinkled bloodshot eyes. "I don't want to live in a zoo on an alien planet. I want to go home and prove my theory that this problem has a solution."

I grunted my disgust.

"The solution is simple," he said. "We're in a trap so strong that the aliens didn't establish any means to control our actions. When men put a lion in a strong cage, they don't worry about controlling the lion because the lion can't get out. We're in the same basic situation."

"So what?" Verana queried in a sarcastic tone.

"The aliens want us transported to their planet so they can examine and question us. Right?"

"Right."

"Ed, remember that remark the machine made last night?"

"What remark?"

"It said, 'My masters will be displeased with me if you arrive in a damaged condition.' What does that indicate to you?"

I assumed a baffled expression. I didn't have the slightest idea of what he was driving at and I told him so.

"Ed," he said, "if you could build an electronic brain capable of making decisions, how would you build it?"

"Hell, I don't know," I confessed.

"Well, if I could build an electronic brain like the one running this ship, I'd build it with a conscience so it'd do its best at all times."

"Machines always do their best," I argued. "Come on, untie us. I'm getting a crick in my back!" I didn't like the idea of being slugged while asleep. If Kane had been sober and if his wife hadn't been present, I would have let him know exactly what I thought of him.

"Our machines always do their best," he argued, "because we punch buttons and they respond in predetermined patterns. But the electronic brain in this ship isn't automatic. It makes decisions and I'll bet it even has to decide how much energy and time to put into each process!"

"So what?"

He shrugged muscular shoulders. "So this ship is operated by a thinking, conscientious machine. It's the first time I've encountered such a machine, but I think I know what will happen. I spent hours last night figuring—"

"What are you talking about?" I interrupted. "Are you so drunk that you don't know—"

"I'll show you, Ed."

He walked around the table and stood behind my chair. I felt his thick fingers around my throat and smelled the alcohol on his breath.

"Can you see me, machine?" he asked the empty air.

"Yes," the electronic brain replied.

"Watch!"

Kane tightened his fingers around my throat.

Verana and Marie screamed shrilly.

My head seemed to swell like a balloon; my throat gurgled painfully.

"Please stop," the machine pleaded.

"What will your masters think of you if I kill all of us? You'll return to them with a cargo of dead people!"

The machine didn't answer. I waited for the electronic brain to interfere and, with a cold knot in my stomach, realized the machine had said it had no way to control our actions!

"Your purpose won't be fulfilled, will it?" Kane demanded. "Not if you return with dead specimens!"

"No," the machine admitted.

"If you don't take us back to the Moon," Kane threatened, "I'll kill all of us!"

The alien electronic brain was silent.

By this time, I couldn't see and Kane's voice was a hollow, faraway thing that rang in my ears. I tugged at my bindings, but they only tightened as I struggled.

"If you take us back to the Moon, your masters will never know you failed in your mission. They won't know you failed because you won't bring them proof of your failure."

My fading consciousness tried to envision the alien mechanical brain as it struggled with the problem.

"Look at it this way," Kane persisted. "If you carry our corpses to your masters, all your efforts will have been useless. If you return us to the Moon alive, you'll still have a chance to carry out your mission later."

A long silence followed. Verana and Marie screamed at Kane to let go. A soft darkness seemed to fill the room, blurring everything, drowning even their shrieks in strangling blackness.

"You win," the machine conceded. "I'll return the ship to the Moon."

Kane released his grip on my throat.

"See?" he asked. "Didn't I tell you every problem has a solution?"

I didn't answer. I was too busy enjoying breathing again.