I

Our ship, the space-flyer, Planetara , whose home port was Greater New York, carried mail and passenger traffic to and from both Venus and Mars. Of astronomical necessity, our flights were irregular. The spring of 2070, with both planets close to the Earth, we were making two complete round trips. We had just arrived in Greater New York, one May evening, from Grebhar, Venus Free State. With only five hours in port here, we were departing the same night at the zero hour for Ferrok-Shahn, capital of the Martian Union.

We were no sooner at the landing stage than I found a code flash summoning Dan Dean and me to Divisional Detective Headquarters. Dan "Snap" Dean was one of my closest friends. He was electron-radio operator of the Planetara . A small, wiry, red-headed chap, with a quick, ready laugh and the kind of wit that made everyone like him.

The summons to Detective-Colonel Halsey's office surprised us. Dean eyed me.

"You haven't been opening any treasure vaults, have you, Gregg?"

"He wants you, also," I retorted.

He laughed. "Well, he can roar at me like a traffic switch-man and my private life will remain my own."

We could not think why we should be wanted. It was the darkness of mid-evening when we left the Planetara for Halsey's office. It was not a long trip. We went on the upper monorail, descending into the subterranean city at Park Circle 30.

We had never been to Halsey's office before. Now we found it to be a gloomy, vaultlike place in one of the deepest corridors. The door lifted.

"Gregg Haljan and Daniel Dean."

The guard stood aside. "Come in."

I own that my heart was unduly thumping as we entered. The door dropped behind us. It was a small blue-lit apartment—a steel-lined room like a vault.

Colonel Halsey sat at his desk. And the big, heavy-set, florid Captain Carter—our commander on the Planetara —was here. That surprised us: we had not seen him leave the ship.

Halsey smiled at us gravely. Captain Carter spoke with an ominous calmness: "Sit down, lads."

We took the seats. There was an alarming solemnity about this. If I had been guilty of anything that I could think of, it would have been frightening. But Halsey's words reassured me.

"It's about the Grantline Moon Expedition. In spite of our secrecy, the news has gotten out. We want to know how. Can you tell us?"

Captain Carter's huge bulk—he was about as tall as I am—towered over us as we sat before Halsey's desk. "If you lads have told anyone—said anything—let slip the slightest hint about it...."

Snap smiled with relief; but he turned solemn at once. "I haven't. Not a word!"

"Nor have I!" I declared.

The Grantline Moon Expedition! We had not thought of that as a reason for this summons. Johnny Grantline was a close friend of ours. He had organized an exploring expedition to the Moon. Uninhabited, with its bleak, forbidding, airless, waterless surface, the Moon—even though so close to the Earth—was seldom visited. No regular ship ever stopped there. A few exploring parties of recent years had come to grief.

But there was a persistent rumor that upon the Moon, mineral riches of fabulous wealth were awaiting discovery. The thing had already caused some interplanetary complications. The aggressive Martians would be only too glad to explore the Moon. But the United States of the World, which came into being in 2067, definitely warned them away. The Moon was Earth territory, we announced, and we would protect it as such.

There was, nevertheless, a realization by our government, that whatever riches might be upon the Moon should be seized at once and held by some reputable Earth Company. And when John Grantline applied, with his father's wealth and his own scientific record of attainment, the government was glad to grant him its writ.

The Grantline Expedition had started six months ago. The Martian government had acquiesced to our ultimatum, yet brigands have been known to be financed under cover of a government disavowal. And so our expedition was kept secret.

My words need give no offence to any Martian who comes upon them. I refer to the history of our Earth only. The Grantline Expedition was on the Moon now. No word had come from it. One could not flash helios even in code without letting all the universe know that explorers were on the Moon. And why they were there, anyone could easily guess.

And now Colonel Halsey was telling us that the news was abroad! Captain Carter eyed us closely; his flashing eyes under the white bushy brows would pry a secret from anyone.

"You're sure? A girl of Venus, perhaps, with her cursed, seductive lure! A chance word, with you lads befuddled by alcolite?"

We assured him that we had been careful. By the heavens, I know that I had been. Not a whisper, even to Snap, of the name Grantline in six months or more.

Captain Carter added abruptly, "We're insulated here, Halsey?"

"Yes. Talk as freely as you like. An eavesdropping ray will never get through to us."

They questioned us. They were satisfied at last that, though the secret had escaped, we had not given it away. Hearing it discussed, it occurred to me to wonder why Carter was concerned. I was not aware that he knew of Grantline's venture. I learned now the reason why the Planetara , upon each of her last voyages, had managed to pass fairly close to the Moon. It had been arranged with Grantline that if he wanted help or had any important message, he was to flash it locally to our passing ship. And this Snap knew, and had never mentioned it, even to me.

Halsey was saying, "Well, apparently we can't blame you: but the secret is out."

Snap and I regarded each other. What could anyone do? What would anyone dare do?

Captain Carter said abruptly, "Look here, lads, this is my chance now to talk plainly to you. Outside, anywhere outside these walls, an eavesdropping ray may be upon us. You know that? One may never even dare to whisper since that accursed ray was developed."

Snap opened his mouth to speak but decided against it. My heart was pounding.

Captain Carter went on: "I know I can trust you two more than anyone under me on the Planetara ."

"What do you mean by that?" I demanded. "What—"

He interrupted me. "Just what I said."

Halsey smiled grimly. "What he means, Haljan, is that things are not always what they seem these days. One cannot always tell a friend from an enemy. The Planetara is a public vessel. You have—how many is it, Carter?—thirty or forty passengers this trip tonight?"

"Thirty-eight," said Carter.

"There are thirty-eight people listed for the flight to Ferrok-Shahn tonight," Halsey said slowly. "And some may not be what they seem." He raised his thin dark hand. "We have information...." He paused. "I confess, we know almost nothing—hardly more than enough to alarm us."

Captain Carter interjected, "I want you and Dean to be on your guard. Once on the Planetara it is difficult for us to talk openly, but be watchful. I will arrange for us to be doubly armed."

Vague, perturbing words! Halsey said, "They tell me George Prince is listed for the voyage. I am suggesting, Haljan, that you keep your eye especially on him. Your duties on the Planetara leave you comparatively free, don't they?"

"Yes," I agreed. With the first and second officers on duty, and the Captain aboard, my routine was more or less that of an understudy.

I said, "George Prince? Who is he?"

"A mechanical engineer," said Halsey. "An underofficial of the Earth Federated Catalyst Corporation. But he associates with bad companions—particularly Martians."

I had never heard of this George Prince, though I was familiar with the Federated Catalyst Corporation, of course. A semigovernment trust, which controlled virtually the entire Earth supply of radiactum, the catalyst mineral which was revolutionizing industry.

"He was in the Automotive Department," Carter put in. "You've heard of the Federated Radiactum Motor?"

We had, of course. It was a recent Earth discovery and invention. An engine of a new type, using radiactum as its fuel.

Snap demanded, "What in the stars has this got to do with Johnny Grantline?"

"Much," said Halsey quietly, "or perhaps nothing. But George Prince some years ago mixed in rather unethical transactions. We had him in custody once. He is known as unusually friendly with several Martians in Greater New York of bad reputation."

"Well?"

"What you don't know," Halsey said, "is that Grantline expects to find radiactum on the Moon."

We gasped.

"Exactly," said Halsey. "The ill-fated Ballon Expedition thought they had found it on the Moon, shortly after its merit was discovered. A new type of ore—a lode of it is there somewhere, without doubt."

He added vehemently, "Do you understand now why we should be suspicious of this George Prince? He has a criminal record. He has a thorough technical knowledge of radium ores. He associates with Martians of bad reputation. A large Martian company has recently developed a radiactum engine to compete with our Earth motor. There is very little radiactum available on Mars, and our government will not allow our own supply to be exported. What do you suppose that company on Mars would pay for a few tons of richly radioactive radiactum such as Grantline may have found on the Moon?"

"But," I objected, "That is a reputable Martian company. It's backed by the government of the Martian Union. The government of Mars would not dare—"

"Of course not!" Captain Carter exclaimed sardonically. "Not openly! But if Martian Brigands had a supply of radiactum I don't imagine where it came from would make much difference. The Martian company would buy it, and you know that as well as I do!"

Halsey added, "And George Prince, my agents inform me, seems to know that Grantline is on the Moon. Put it all together, lads. Little sparks show the hidden current.

"More than that: George Prince knows that we have arranged to have the Planetara stop at the Moon and bring back Grantline's ore.... This is your last voyage this year. You'll hear from Grantline this time, we're convinced. He'll probably give you the signal as you pass the Moon on your way out. Coming back, you'll stop at the Moon and transport whatever radiactum ore Grantline has ready. The Grantline Flyer is too small for ore transportation."

Halsey's voice turned grimly sarcastic. "Doesn't it seem queer that George Prince and a few of his Martian friends happen to be listed as passengers for this voyage?"

In the silence that followed, Snap and I regarded each other. Halsey added abruptly:

"We had George Prince typed that time we arrested him four years ago. I'll show him to you."

He snapped open an alcove, and said to his waiting attendant "Flash on the type of George Prince."

Almost at once, the image glowed on the grids before us. He stood smiling sourly before us as he repeated the official formula:

"My name is George Prince. I was born in Greater New York twenty-five years ago."

I gazed at this televised image of George Prince. He stood somber in the black detention uniform, silhouetted sharply against the regulation backdrop of vivid scarlet. A dark, almost femininely handsome fellow, well below medium height—the rod checking him showed five foot four inches. Slim and slight. Long, wavy black hair, falling about his ears. A pale, clean-cut, really handsome face, almost beardless. I regarded it closely. A face that would have been beautiful without its masculine touch of heavy black brows and firmly set jaw. His voice as he spoke was low and soft; but at the end, with the concluding words, "I am innocent!" it flashed into strong masculinity. His eyes, shaded with long girlish black lashes, by chance met mine. "I am innocent." His curving sensuous lips drew down into a grim sneer....

Halsey snapped a button. He turned back to Snap and me as his attendant drew the curtain, hiding the black grid.

"Well, there he is. We have nothing tangible against him now. But I'll say this: he's a clever fellow, one to be afraid of. I would not blare it from the newscasters' stadium, but if he is hatching any plot, he has been too clever for my agents!"

We talked for another half-hour, and then Captain Carter dismissed us. We left Halsey's office with Carter's final words ringing in our ears. "Whatever comes, lads, remember I trust you...."

Snap and I decided to walk part of the way back to the ship. It was barely more than a mile through this subterranean corridor to where we could get the vertical lift direct to the landing stage.

We started off on the lower level. Once outside the insulation of Halsey's office we did not dare talk of this thing. Not only electrical ears, but every possible eavesdropping device might be upon us. The corridor was two hundred feet or more below the ground level. At this hour of the night the business section was comparatively deserted. The stores and office arcades were all closed.

Our footfall echoed on the metal grids as we hurried along. I felt depressed and oppressed. As though prying eyes were upon me. We walked for a time in silence, each of us busy with memory of what had transpired at Halsey's office.

Suddenly Snap gripped me. "What's that?"

"Where?" I whispered.

We stopped at a corner. An entryway was here. Snap pulled me into it. I could feel him quivering with excitement.

"What is it?" I demanded in a whisper.

"We're being followed. Did you hear anything?"

"No!" Yet I thought now that I could hear something. Vague footfalls. A rustling. And a microscopic whine, as though some device were within range of us.

Snap was fumbling in his pocket. "Wait! I've got a pair of low-scale detectors."

He put the little grids against his ears. I could hear the sharp intake of his breath. Then he seized me, pulled me down to the metal floor of the entryway.

"Back, Gregg! Get back!" I could barely hear his whisper. We crouched as far back into the doorway as we could get. I was armed. My official permit for the carrying of the pencil heat ray allowed me always to have it with me. I drew it now. But there was nothing to shoot at. I felt Snap clamping the grids on my ears. And now I heard something! An intensification of the vague footsteps I had thought I heard before.

There was something following us! Something out in the corridor there now! The corridor was dim, but plainly vis ible, and as far as I could see it was empty. But there was something there. Something invisible! I could hear it moving. Creeping toward us. I pulled the grids off my ears.

Snap murmured, "You've got a local phone?"

"Yes. I'll get them to give us the street glare!"

I pressed the danger signal, giving our location to the operator. In a second we got the light. The street in all this neighborhood burst into a brilliant actinic glare. The thing menacing us was revealed! A figure in a black cloak, crouching thirty feet away across the corridor.

Snap was unarmed but he flung his hands out menacingly. The figure, which may perhaps not have been aware of our city safeguard, was taken wholly by surprise. A human figure, seven feet tall at the least, and therefore, I judged, a Martian man. The black cloak covered his head. He took a step toward us, hesitated, and then turned in confusion.

Snap's shrill voice was bringing help. The whine of a street guard's alarm whistle nearby sounded. The figure was making off! My pencil ray was in my hand and I pressed its switch. The tiny heat ray stabbed through the air, but I missed. The figure stumbled but did not fall. I saw a bare gray arm come from the cloak, flung up to maintain its balance. Or perhaps my pencil ray had seared his arm. The gray-skinned arm of a Martian.

Snap was shouting, "Give him another!" But the figure passed beyond the actinic glare and vanished.

We were detained in the turmoil of the corridor for ten minutes or more with official explanations. Then a message from Halsey released us. The Martian who had been following us in his invisible cloak was never caught.

We escaped from the crowd at last and made our way back to the Planetara , where the passengers were already assembling for the outward Martian voyage.