The muffled, helmeted figure of a pilot climbed down the spider ladder, nestled into the foremost scout’s cockpit and pressed the starting button. The motor spat out a wisp of smoke, then burst into its full-throated roar: the automatic clamp above loosened: the scout dropped plummet-like, bobbed to the flagship below, straightened out and zoomed six thousand feet up into the morning blue, where it hovered for a few moments like an eagle on taut wings. Lieutenant Christopher Travers, the pilot, glanced around.

Behind and below him was spread a magnificent panorama. Across the plate of scintillating glass that was the sea moved rows of toy ships, tipped by the gleaming, one-fifth-mile long shape of a dirigible, of whose three scout planes Chris’s was the leader. As he watched, the second scout dropped from the plane rack beneath the dirigible’s sleek underside and went streaking away, followed by the third, in response to the Admiral’s order of: “Proceed ahead to locate the enemy’s position.”

A grin relaxed Chris Travers’ tanned, boyish face. His narrowed gray eyes swept the horizon. Below it somewhere lay hidden the ranks of the Black Fleet, complete with its own destroyers, submarines, cruisers, battleships, aircraft carriers and the ZX-2, sister dirigible of the Blue Fleet’s ZX-1. Chris spurted the scout ahead and murmured:

“This war game’s goin’ to be a big affair--the biggest yet!”

It was. The Atlantic Fleet of the United States Navy, termed “Blue” for convenience, had been assigned to guard the Panama Canal; the Pacific Fleet, “Black,” to attack it. The cream of America’s sea forces had been assembled for that week of March, 1935, all the way from crabby little destroyers to the two newly completed monarchs of the air, the twin dirigibles, fresh from the hangars at Akron, a thousand feet each in length and loaded with the latest offensive and defensive devices developed by Government laboratories.

[Illustration]

The war game around the Canal was planned for more than practice, however. The eyes of the whole world were on that array of America’s ocean might--the eyes of one foreign nation in particular. Washington knew of the policies of that nation, and wished to impress upon is the hopelessness of them. More than a game, this concentration of sea and air-borne fighting power was a gesture for the continued peace of the world--a gesture strong with the hint of steel.

Chris Travers was vaguely aware, through the rumors of the mess-room, of the double meaning of the game he was playing his part in, but this morning he didn’t give a single thought. He was too wrapped up in his job of spotting the van of the Black Fleet, radio-telephoning latitude and longitude to the bridge of the Blue Fleet flagship, and getting home to his dirigible without being declared destroyed by one of the war game umpires.

Therefore, half an hour later, his heart thrilled as he glimpsed, wraith-like on the steely horizon, a wisp of smoke.

He catapulted forward, eyes steady on that hint of ships. The smoke grew to a cloud of black pouring from the funnels of a V-shaped squad of destroyers, rolling through the lazy swells of the Pacific waters. Behind them came the bulldogs, larger warships, hazy blurs in the distance.

Chris struck fist in palm to the tune of a gleeful chortle. He was first! He hauled the microphone from its cubby in the dashboard and spoke the code words. Latitude, longitude and steaming direction of the Black Fleet he gave rapidly, and the information knifed back to the bridge of the Blue Fleet flagship, a hundred miles behind, where a white-haired admiral said: “Ah! Good boy! Get those bombers up--pronto!”

Chris commanded a superb view of the ZX-2, whose gleaming shape, showering rays of sunlight, hung like a thing in a painting over the Black Fleet. He stared at the far-off dirigible, lost in admiration of her trim lines, pausing a minute before returning to his own ZX-1. At that distance, the mammoth craft seemed no more than four inches long, yet, through his telescopic sight, he could discern her markings, machine-gun batteries and the airplane rack along her belly plainly. One plane, he saw, was suspended from the rack; the others were scouting for the Blue Fleet, even as he had scouted for the Black. He wondered if something were wrong with the plane left behind. Somehow, it did not look quite familiar.

But, even as he watched, it dropped from the automatic rack, then straightened and soared dizzily up. And, from one of the airplane carriers’ broad decks, he saw two pursuit craft begin to rise. He grinned. They’d seen him, were coming after him!

He gripped the stick, prepared to swerve around. He had already raised a spread-fingered hand for a derisive parting gesture, when suddenly he stiffened. The hand dropped as if paralyzed.

“Good Lord!” he gasped. “What--”

The mighty thousand-foot dirigible ZX-2, pride of the Navy and all America, had wobbled drunkenly in her path. She stuck her nose down, and then her whole vast frame shivered like a wind-whipped leaf as the dull roar of an explosion rolled over the sea. A huge sliver of hide was stripped from her as if by magic, revealing the skeleton of girders inside--revealing a tongue of crimson that licked out and welled into a hell of flame.

Chris’s blood froze. He watched the ZX-2 wallow in her death throes, writhe in the fiery doom that had struck her in seconds, that was devouring her with awful rapidity while thousands of men, blanched and trembling, gazed on helplessly. He saw her plunge, a blazing inferno, into the sea beneath...

There were old pals on her--buddies, gone in a flash of time!

This wasn’t a war game. This was tragedy, stark before his eyes.

The Black Fleet forgot its mimic battle. Radio telephone messages winged over the horizon to the approaching Blue Fleet. The Black dreadnoughts hove to; launches with ashen-faced men in white manning them dropped overboard; a dozen destroyers rolled in the swells around a crumbled, charred egg-shell that but minutes before had been an omnipotent giant of the sky.

Chris Travers, aloft in sunlight suddenly bereft of its beauty, jammed the stick of the scout full over. He could do nothing, he knew. He could only return to the ZX-1 and tell the story of its sister as he had seen it.

But why, he wondered as he flew almost blindly, had the ZX-2 so quickly flamed to oblivion? The helium of its inner bags bad been uninflammable, as had the heavy oil of its fuel tanks; the ten engines were Diesels, and hence without the ordinary ignition system and gasoline. Safety devices by the score bad been installed on board; nothing had been overlooked. And the weather, perfect.

It was uncanny. It seemed totally unexplainable.

Swarms of planes droned between sea and sky, all speeding in the one direction, west, to where the crumpled remnants of a dirigible were slipping quickly beneath the billows, beyond the sight of man. Planes of war game umpires, of officials, of newspaper correspondents and photographers. And soon a spectral, gleaming wisp of silver nosed out of the east, and the lone scout flying east dropped in altitude to meet its mother.

Mechanically, his mind elsewhere, Chris shoved the button which reared the automatic clamp behind the cockpit in preparation for affixing the scout to the plane rack beneath the ZX-1. The dirigible, far in advance of the Blue Fleet, was roaring along at its full one hundred and fifty to hover over the grave of its sister. Chris eyed its course and changed his. To jockey into the rack, he had to pass the dirigible and come up underneath from its rear.

The air giant roared closer. As the distance between then loosened, Chris’s brow wrinkled and he swore softly in puzzlement.

“Now, just what’s wrong with them?” he exclaimed, “The darned zep isn’t flying straight! She’s wobbling in her course!”

It was hardly apparent, but true. Ever so slightly, the snub nose of the ZX-1 was swaying from side to side as it sped through the air; ever so slightly, her massive stern directional-rudders were wavering.

She was less than a mile away now. At that time, there were no other planes in sight; none flying in that vicinity save Chris’s. He glued his eyes to the telescopic sight. A moment later, sheer horror swept his face.

“Good God!“

The scout leaped as its throttle rammed down. The gleaming, thousand-foot shell of the ZX-1 roared by it at equal altitude, making it a puny fly-speck in the sky. But the fly-speck was faster. It turned in a screaming bank; it straightened; it lunged back after the swaying, retreating mammoth like a whippet, lower, now, than its quarry. It maneuvered expertly as it gained, for one of the best pilots of the service was at its controls, and there were deep lines graven in his face, lines of anguish and intolerable suspense.

Through the telescopic sight, Chris had not seen a single white-clad figure standing beside the glass ports of the dirigible’s control car. But he had seen, slung from the rack along her belly, a single plane--the same rather peculiar-looking plane he had seen hanging beneath the rack of the ZX-2 a few minutes before she had gone down in flames!

And in that plane, he knew surely, was the answer to the mystery.

Speed cut to just a trifle more than the dirigible’s. Chris passed a few feet underneath the huge expanse of her lower directional rudder. From so close, its uncontrolled wavering was terrifying.

His faculties were concentrated on the task of sliding the scout’s clamp into the groove of the plane rack, but he was also surveying the lone airplane hanging from it. A powerful machine, painted in Navy colors, a peculiar knob on the upper side of each half of the top wing gave it its unfamiliar appearance. Its pilot was obviously aboard the dirigible, working...

Closer and closer the scout crept, quarter-way now along from the stern of the massive bulk that loomed above it, and within fifty feet of the third clamp in the rack. Touchy work, maneuvering into it, with the ZX-1 yawing as she was, and the need for haste desperate. Chris’s hands were glued to the stick: his nerves were as tight as violin strings. Then, when only ten feet from the rack clamp, he gave a startled jump of uncomprehending amazement.

The propeller of the mysterious plane ahead had roared over. Its clamp had left the rack; it had dropped down in a perfectly controlled dive and flattened out as if a master pilot were at its controls.

But the plane’s cockpit was still empty, Chris could see; nor had he seen any figure pass down the ladder from the dirigible into it!

Devoid of all emotion save bewilderment, he sat stupidly in the scout. A moment later, so well had he aimed it, its clamp nestled snugly into the groove of the rack, and the regular automatic action took place. A tiny door slid open directly above in the dirigible’s hull: a thin ladder craned down--and Chris’s nostrils caught a faint whiff of something that cleared his mind of its confusion instantly.

Just a whiff, but it registered. Gas, with an odor resembling carbon monoxide.

He stared up. Over the edge of the automatic trap-door above, a white, contorted face was hanging. The dirigible swung; white-clad shoulders and body slumped into view. Then, with a rush, the body slipped through, jarred against the connecting ladder, slithered off and went twisting and turning into the gulf below.

“God!”

Gassed! How, by what, Chris had no idea. A moment before he had been about to follow the uncannily piloted plane; but now his duty was plain. He knew with awful certainty that in minutes, seconds perhaps, the giant ZX-1 was scheduled to roar into flames like its sister and plunge into the Pacific.

He jerked out a gas mask. He was fitting it on with one hand as, with the other, he hauled himself up the spider ladder into the hull of the thundering, yawing dirigible.

He did not see, hovering a few hundred yards behind the ZX-1, the mystery plane; he did not see it now begin to approach the rack once more.

The crew of that dirigible of death, Chris discovered, had not had a chance. White-clad bodies lay sprawled throughout the cabin which contained the mechanism of the plane rack, stricken down silently at their posts. There was no life, no sound save the booming of the motors and the whip of the wind screaming past the uncontrolled air titan.

But he did not pause there. He did not know what he was grappling with--it seemed black magic--but he darted to a ladder which angled up from the lowermost entrance cabin to the cat-walk that stretched from the nose to the stern of the ship. If any infernal contrivance had been planted aboard, it would be in the most vital spot.

Heart pumping from the artificial air he was breathing and from the consciousness that each second might well be his last, he sprinted along the interior gangway. Above was the vasty gloom of the gas bags and the interweaving latticework of the supporting girders; the drum of power-car motors and the strained creakings of cables and supports echoed weirdly throughout. Outside was the sun and the sea and the clean air, but this realm of mammoth shapes and dimness seemed apart from the world. Once he stumbled against something soft and yielding--a body flung down there in death, fingers at its throat. And there were other white-clad figures, grimly marking off the length of the cat-walk...

Chris’s nerves were raw and his face sopping with sweat beneath its mask when suddenly he stopped at sight of something that lay on the cat-walk, with the main fuel tanks on the girders just above it and the entrance to the control car just below.

It was a black box, perhaps two feet square and a foot in depth, made of dull metal that did not reflect the rays of the light bulb placed at the head of the ladder leading down in the control car. There were three curious little dials on its face, and the trembling finger of each one was mounting.

It had been strategically placed. An explosion at that point would rip open the fuel tanks, split the largest gas bag, wreak havoc on an intricate cluster of main girders, and destroy the control car with its mechanism.

“No wonder the ZX-2 crashed!” Chris muttered.

Then his hands swept down. The next instant he was hugging the thing tight to his chest and stumbling down into the control car, hearing only a high-pitched, impatient whine that was coming from the box as the fingers of its dials crept slowly upward.

The ZX-1 was wavering wildly as her rudders flopped from side to side, and with every swing the bodies that lay in her control car, strangled by gas, stirred slightly. The gray-haired commander was stretched there, one arm limply rolling as his ship, which had gone so suddenly from him, rolled. Subordinate officers were tumbled around him. Death rode the control car.

But down to it and through it now came one who was alive, a figure made grotesque by the mask it wore and the pack of the parachute strapped to it, who threaded past the littered bodies, an ever-rising whine wailing from the box clasped in his arms.

With a leap, he was at one of the car’s port-holes, fingers fumbling at the heavy bolts. The seconds seemed eternal, and the box’s whine had become a shattering, sinister scream when at last the bolts loosened. The round pane of glass teetered back, swung open--and the masked man slung his metal burden out, out from the ZX-1 into the gulf between sea and sky.

It arced through the sunlight, went spinning down, became a dot, its screaming faded. Then something synchronized within it, and it was gone--in a burst of weird, bluish light, whose fangs forked upwards for a second, their unearthly flash dimming even the sunlight, and then were gone, too...

Chris found that his whole body was shaking. For a moment he stood there with his masked face through the port.

“Damn close,” he muttered. “But what was it that left the box here?”

Then he jarred against the side of the car as the ship swung and came back to realization of what was needed to be done, and done at once. He shifted his gaze, drew his head back, and thrust it forth again, staring.

“Good Lord!” he cried. “That plane’s come back!”

His own craft was not alone under the rack. The same mysterious machine hung there again, its cockpit empty, and the automatic spider ladder was stretched down to it from the trap-door in the dirigible above.

“Whatever flies it is aboard now.” Chris thought aloud. “But it got back too late to stop me. Well, this time--”

He felt uneasy, however, almost powerless. What was this thing that had wiped out the crews of two dirigibles with deadly gas, and wrecked one of them? He spun around. The control car looked the same. But what might be moving in it?...

Chris carried no gun; but he extracted the service repeater from the holster of a body at his feet. Gripping it, he leaped to the helm of the dirigible. It was the work of a moment to clamp on the mechanical “iron mike,” which steadied the ZX-1’s mad swaying and leveled her ahead in a dead straight course. He could not cut down her speed, unless he went to each one of the hull-enclosed engine stations, and more urgent work awaited before he could afford to do that--work of sending out an S.O.S. before the weird, unseen killer and wrecker came to grips with him.

Though seeming hours, only minutes had passed since he had tooled his scout into the rack. Ahead, he could see the smudge of the Black Fleet’s smoke on the horizon. Not so very far away, but a lot could happen in the distance still separating dirigible and surface craft.

He ran back into the radio-telephone cubby, which was a division of the control car. The operator was sprawled there, limp in his seat before the shining, switch-studded panel. Chris removed the head-gear of ear-phones: then he hauled one of the cubby’s port-holes open, letting in a rush of cleansing air. His fingers sped quickly over the panel; a row of tubes glowed; the machinery hummed. Chris jerked off his mask.

A last faint odor was present, but he hardly noticed it, for his lips were at the mouthpiece and he was thrusting out a call for help.

“ZX-1 calling ... ZX-1 calling ... ZX-1--Hello!”

An answer from the flagship of the Black Fleet ahead had sounded.

“This is Travers, pilot on the ZX-1, speaking. We’re coming dead for you; full speed; you’ll see us in minutes. Get some planes with men capable of handling the dirigible up here immediately. The whole crew’s been laid out by gas; there was a contrivance planted aboard to blow up the ship and send it down in flames as the ZX-2 was. The thing that did it--”

Crack!

A gun barked out from behind; something crashed and splintered on the radio panel. Chris felt a white-hot needle sear along the side of his head. His brain reeled; with everything dancing queerly before him in splotches of gray and black he toppled down off the seat, knowing the radio-telephone had been put out of commission by the cessation of sound in the ear-phones clamped to him.

He gripped his consciousness hard. It was like a delirium: he was lying sprawled beside the seat, twisted round so that he saw, hanging in the cubby’s entrance door, an automatic, dribbling a wisp of smoke--the automatic that had just fired, but hanging there by itself, held by something he could not see!

He was only half conscious, for the scorching pain along his head was throbbing his brain dizzily, but he realized that the service repeater he had taken from the control car lay by his side, within easy reach. But, while on the verge of risking a wild grab for it, he heard a voice, speaking very softly and with a slight thickness of accent.

“Do not move,” it said. “I fire if you do. Now, listen: What did you do with the box that you found? Tell me quick, or die.”

It was fantastic, unreal. There was--nothing, and yet a man, living, breathing, but invisible, was speaking! Chris could not understand; but it was at least a little relief to know he had a human to deal with. For with humans, strategy can be used...

He groaned. He saw plainly that the unseen marauder had been aboard when he had thrown the box over, and thus had not seen it explode in midair: did not know whether it had been tossed out or merely rendered harmless by being tampered with. If only the latter, it could be quickly repaired and set again. That must be the invisible man’s reasoning.

Again Chris groaned. He moved an arm weakly and whispered:

“Can’t speak much. Come closer.”

The service repeater was very close now to his right hand. And he felt a thrill when he saw the automatic come forward through the air, descend, and pause right next to his head. He sensed a man close behind him, and he heard:

“Well? Tell me, quick. Did you throw it over, or--?”

“Don’t shoot!” Chris groaned. “I’ll tell you. I didn’t--throw it over. I took it apart to get the secret of it. I put it--there.”

He pointed feebly with his right hand, thus leading the invisible man to turn his head. His legs braced imperceptibly. And then:

“Like hell!” roared Chris Travers, and shot his whole weight backwards, grasping the service gun, whipping it around and yanking the trigger three times at the same instant.

Shooting at nothing! But, even above the bunched roar of the explosions, there pierced out a howl of agony that died quickly to a sobbing moan. Chris saw the automatic drop to the floor, felt the invisible body he had crashed into jerk away. He jumped to his feet, clutched at that body, and caught thin air. He swung around, listening, the service repeater in his hand.

Out of the air somewhere before him there came the sound of low, racking gasps, and also the slow noise of feet dragging heavily towards the cubby’s door, towards the ladder that led up to the fore-and-aft cat-walk.

Chris sprang, slashing the butt of the gun downwards. The lead was false. He hurtled jarringly into the door jamb, the gun thumping against the floor. The wind was knocked from him; the nausea of his wound swept him again with a surge of dizziness. But the painful scuffle of unseen feet ahead pulled him up once more; like a punch-drunk fighter he staggered out from the cubby to the ladder and hauled himself up the steps. He half-fell at the top, but his mind was clearing; and as he swayed there he knew what he had to do--saw the duty that lay before him...

More slowly, he crawled after the dragging footsteps and the gasps of the invisible raider, following them through the vast dimness of the interior of the dirigible ZX-1.

The chief operator on duty in the flagship of the Black Fleet swung round in his seat and yelled through into the bridge of the massive battleship:

“Urgent, sir! From the ZX-1!”

A moment later the captain of the ship, for the fleet’s admiral was out in a launch inspecting what little of the fallen ZX-2 was still floating on the surface, was at the operator’s side, listening amazedly.

The operator read off, word for word, what Chris Travers had sent. “ ... There was a contrivance planted aboard to blow up the ship and send it down in flames as the ZX-2 was. The thing that did it is--” he finished, and fell silent on that uncompleted sentence.

The captain’s lined face expressed incredulity. “My God!” he burst out. “First the ZX-2, now-- That all?”

“Yes, sir. I can’t get any answer or connection.”

They stared at each other. Finally the captain spluttered:

“Is some maniac loose in this fleet? Don’t sit there like a fool, man! Get in touch with the Saratoga; tell ‘em what you received; tell ‘em to send some men up to that dirigible, wherever she is. We can’t lose both of them!”

The operator’s fingers skipped nimbly; even while he was speaking into the microphone, the red-faced captain had rushed back into the control bridge and was roaring:

“Signal the Admiral back here! Hurry!”

Things moved quickly then; small things, but significant. A casual eye glancing over the ranks of the Black Fleet as it lay around the scene of the tragedy, waiting for orders, would not have noticed any difference. The launch containing the fleet’s admiral, which had been fussing about with its load of officers and various dignitaries, suddenly wheeled and pointed back for the mammoth flagship, in response to swift signals from the arms of a gob on her bridge; and, on the broad landing deck of the carrier, Saratoga, two three-seater planes, equipped with automatic clamps for a dirigible’s rack, were wheeled up to the line.

Their props were spun over. But even before their cockpits had been filled, an officer on the bridge of the flagship, and a dozen others throughout the fleet, cried:

“There she is!”

Over the eastern horizon, a gleaming sliver in the sunlight, thundered the ZX-1, straight for the array of the Black Fleet. Only a few men were aware of the drama-fraught message which had come down from her radio cubby, but her growing shape commanded the eyes of every sailor and officer alike who had time to watch. A few telescopic sights were trained on her as she bellowed ahead; the keen old eyes of a very perplexed and puzzled admiral were at one of them.

“Two planes hanging from her rack,” he muttered, half to himself and half to the officers standing around him. “Both Navy. Say, they’re dropping off! Not coming this way, either. Going northeast. Fast, too. Can’t see ‘em any more ... Those men getting up from the Saratoga? Good. We’ll find out something soon. Here she comes!”

Closer and closer roared the dirigible. Two planes from the Saratoga were swooping up to enter her rack, but the other two planes that shortly before had been suspended from it were gone--already vanished into the northeast.

“Don’t understand this at all!” said the Admiral of the Black, or Pacific, Fleet of the United States Navy.

Things had broken well, Chris Travers considered. He had only wounded the invisible raider; but, luckily, had wounded him badly, so that, evidently, just one object was in the man’s mind: to get back to where he came from, to where he could find help. He seemed oblivious of the scout that was following behind at the full speed of its mighty rotary motor, following him to his base, wherever it was.

“Just as well I didn’t kill him,” Chris muttered.

The rush of wind had cleared his brain; his faculties were steady and normal. Not so with the man in the plane he pursued. It was flying crazily, but clinging to one course, nevertheless--into the northeast, towards land, some two hundred and fifty miles over the horizon.

The great silver shape of the ZX-1, barren, now, of life, dropped away, speeding ever due west; the hazy dots and blur of smoke which denoted the motionless Black Fleet vanished. But Chris was in contact with the fleet’s flagship once more, through the compact radio-telephone set of his scout. As he flew, his eyes fixed steadily on the plane ahead, he was rapping into the microphone the story of what had happened. He told of the invisibility of the strange marauder, of how accurately he had judged the time of his raids; of how he, Chris, had managed to prevent the destruction of the ZX-1.

“He uses a tremendously expansive gas resembling carbon monoxide,” he went on. “It seeps into every cranny of the dirigible, killing everything. The crews got no warning; they didn’t know what was happening; couldn’t see him! Well, I managed to wound him on the ZX-1. He beat it. I’m following him. If he lasts out, he’ll go to where he came from, and we’ll find out who’s in back of all this. Let you know where his base is soon as I get there. Keep listening. Okay? Right; signing off.”

Silence, then, between the scout and the flagship far behind...

On--on Time passed. The scout’s gas was down below the half-way mark. They had covered two hundred miles now at a speed just bordering three hundred. The plane ahead looked uncanny with its apparently empty cockpit, but Chris could see all too well that death was pressing at its invisible pilot. The big machine was literally staggering in its course as the hands on its control stick grew weaker; was yawing wildly, even as the ZX-1 had yawed after her crew had been slain by vapors they could not see.

“He’s got to last out!” Chris muttered. “Got to!”

At that moment land appeared, and the fleeing plane altered its northeast course to due east with an abrupt jerk.

First it was a mere hazy line on the horizon; then it rose to a thrust of land, jutted with cloud-misted hill-tops. Then, as the two roaring specks that were airplanes came closer, heavy tropical foliage became distinct, and white slashes of surf breaking on the shore. This was the Azuero Peninsula, most western point of the Republic of Panama.

Aside from one small cluster of wretched huts, it was practically uninhabited. Guarded by dense growth, only one or two of the dusty paths which passed for roads wandered aimlessly through its tangled creepers, trees and bush. To the southeast was the broad Gulf of Panama, doorway to the Canal; on the other sides this thumb of land was surrounded by the reaches of the Pacific.