Flying low, as though trying to account for the mysteriously twinkling glow from the helicopter, Don watched carefully.

“There’s the crash boat,” he murmured, as his sharp eyes made out the dark object against the sheen of the still water in a channel.

“I’ll ‘give it the gun,’ now,” he decided. “The noise will drown out Garry’s motor hum.”

He opened his throttle.

Necessarily he drew further away. That suited their plan perfectly: it gave him distance in which to turn for his approach in a position to come down in a power-stall that would keep the engine running just fast enough to let the Dart settle onto the water without too much forward speed.

Garry’s hand was on the switch of the stopped electric motor: in the other he held his self-igniting flare.

“The tide will drift me around that clump of eel grass,” he told himself, “then——”

Don, as he saw, was banking around.

“Now!” Garry decided that the time was at hand.

Don was coming in.

The switch was thrown. The engine hummed, and forward drove the fast launch.

Above the swamp Don began his approach.

Surprised by the sudden illumination as Garry’s flare lighted up the small expanse of water where the helicopter lay, the Thing in the disguising oilskins and rubber face mask and cap and gloves turned to strain eyes through small cut holes.

Gathering speed the boat came toward his aerial chariot.

With a yell that seemed to combine anger and dismay the Demon swung to the momentum starter of his motor, sent its disk around feverishly.

Garry’s boat swept alongside, the switch on and gears in reverse to drag it to a stop.

Don, flying in, kept his engine droning, settling toward the water.

It seemed to both Don and Garry that their “Demon” was all but a prisoner.

Garry cut out the switch, allowing the boat to drive close.

“Give up!” he shouted, to be heard above the noise of Don’s approach from the air. “We’ve got you.”

The disguised creature gave a shout of defiance, threw his gears for the revolution that would wake the helicopter engine to life.

“It’s no use!” Garry cried, as Don, perceiving the slow turn of the helicopter’s upper blades as they were rapidly adjusted, lifted the nose and, instead of coming onto the water, stayed aloft, waiting, ready to frustrate any effort to climb away.

Garry’s hand clutched the pontoon braces.

Up he leaped, clinging.

Don saw, in the vivid light, the unexpected thing that happened.

Into the water, on the far side of the helicopter, away from Garry, plunged the queer creature.

Immediately Don cut his power again and dropped the nose; but he had to bank around to get in position once more to power-stall down.

Watching as he executed the maneuver, he saw Garry jump away from his position, half on the helicopter pontoon and half in the boat, turn as he sprang through the air, and strike water, with a flash of lighted spray, to begin swimming with strong, swift strokes, around the end of a pontoon.

Don lost valuable seconds, getting in approach position.

When next he could look, he saw that Garry was at the edge of the heavier concealing clumps of eel grass, treading water, hesitating.

Down came the Dart.

As he took the water Don cut out his power instantly, to enable him to be heard.

“Garry!” he called, “where is he!”

“Hiding in the grass, I guess!”

“Wait!” Don knew that Garry could make no progress trying to swim into that clutching, restraining mesh of tangled grass blades, tall, yielding, but gripping arms and legs in any effort to pass through. “I will get in the crash boat: he’s hiding. The boat will show him—it has a spotlight!”

Garry, treading water, listened for sounds of movement in the clump of grass, agreed.

“You’re captured!” he shouted to the invisible creature. “You might as well give up and save trouble.”

No answer came.

Don saw that the electric-powered boat was almost within reach as he hurriedly unclasped his safety belt, clambered onto a brace of his lightly rocking aircraft, and stretching out his arm, caught hold of the motor boat’s gunwale.

Quickly he got in.

To throw in the switch, and to light the small, but strong, spotlamp in the bow was the work of but a second.

The ray probed across the water, picked out Garry, treading water, close to the grass where he had seen his quarry vanish.

Picking up momentum to the hum of its motor, the vessel under Don’s steermanship moved to a point where Garry could catch its coaming, and draw himself in.

As he deposited his wet body in the bottom, Don backed water, in a slow, curving course, so that the small craft was ready, when he cut out the gears, to be flung forward, with turning room, in any direction he chose. His hand, on the light, swung its beam to and fro, sending scattering, filtered rays through the grass.

“I didn’t hear him move away!” Garry was up and at the bow with Don as he spoke.

“He must be just within the clump of grass.” Don drew closer, at very low speed.

“Listen!” Garry gripped Don’s arm. “Did you hear a shout?”

“Yes! Far away!”

They were distracted, for an instant, from the quest by the new and unexpected call coming from a distant point.

“Do you suppose it could be help—for us?” Garry wondered. “Maybe Chick left the airport to get police aid.”

“Let’s wait a bit and see if the shout comes again!” Don suggested. “This fellow we’re after can’t go far in that grass; he’d sink into a mudhole.”

“Maybe he did, already,” Garry hinted. “Maybe he went down and got caught in the grass.”

“That would account for us not seeing—”

“There’s the call—closer, too!”

They made out the words.

“Sounds like Chick’s voice,” Garry whispered. He called, high and sharp, “Stop!”

“Yes—and there’s a man’s shout—hear him?”

“I’m sure it’s help!” exclaimed Garry.

“But who can they be after?”

“They’re coming closer!”

A movement of the grass caught Don’s attention.

“There’s—” he began.

“No! Listen!” Garry put a hand on his arm, stopping his sentence.

Somewhere not far off, but to the right, inland, the grass clusters seemed to be agitated for a moment.

Don swung the boat, backing to get room, to be ready for the new position of their hidden adversary.

The light swung and focused.

“See anything, Garry? I don’t!”

“No, Don—and I’m sorry you backed the way you did. The helicopter is between us and the place I saw that fellow disappear——”

“But——”

From a point a hundred yards away came a hail

“Hello! You! Showing a light—who are you?”

“Don and Garry—Don McLeod at the helm of the airport crash boat, trying to catch the fellow who has been haunting the swamps and the air.”

“Don—Garry!” Chick’s shrill, excited voice floated to them. “It’s Chick and the control chief! We’re after an Ind——”

The sudden roar of the helicopter engine drowned the last syllable.

Don, reaching for the switch, with the other hand swung his spot beam but it would not swivel far enough to pick out the helicopter’s body.

“I hear somebody in the water!” gasped Garry, “Swing—forward, and swing, Don!”

The launch, in its position facing the left bank of the weed and grass-choked channel, made a difficulty of the forward swing, going too close to the grass. Its propeller caught in the grass or mud.

Instantly Don cut the ignition to avoid losing his propeller.

Garry fled to the stern, bent far, reached down, began disentangling the snagged part.

“Get him!” screeched Garry, to Chick as the light showed him on the bank of a portion of the more solid swamp land at the edge of which the channel ended, far across the sheet of clear water.

From the helicopter there seemed to come a surprised cry, and the sounds of an altercation. There was a splash—but the helicopter went upward!

Don, as the propeller was cleared and Garry shouted that news, did not try to pick up forward speed again. Instead he gave the motor its current with gears in reverse, and backed down toward his own Dart.

“What are you doing that for?”

“Going to the Dart.”

“But the engine is dead.”

“It’s still warm,” Don retorted. “I’m going to bring down that Demon.”

Chick, shouting, appeared at the end of a path, with the control chief, and guided by Doc, who knew the swamp trails.

“There’s somebody swimming!” he screamed.

Don paid no attention.

“Watch him!” he called, and then, as the launch came close to the Dart, caught a wing and clambered into the cockpit.

Garry, who took the wheel, ran forward again until he could revolve the small, fast airplane’s propeller: as his yell was answered by Don’s “contact!” he swerved aside, saw the huge blades begin to swing, heard the roar of the engine, and hastened to get his launch out of the path of the oncoming Dart.

Swiftly gathering speed, with sharp spurts of the gun to clear his choked cylinders, Don lifted his speedy ship into the air, soaring over Garry’s head as the latter, nosing in at the path end, took on Chick.

Up went Don, climbing in as speedy and steep a banked ascent as he dared with a recently stopped power plant to consider. He dared not force the engine until it was again at fullest, safest operating temperature.

The helicopter, rising almost straight up, had an advantage. But Don did not let that concern him. His teeth were tight, clenched, with determination wrinkling his eye-corners.

The Demon had gone too far. Evidently he had meant to lure them in the airplane, only to “finish them off” in some fashion. Garry’s unexpected appearance in the launch had upset those purposes. The Demon, taking advantage of the new chase, getting back to his craft, meant to escape, to lie hidden somewhere, ready to visit more of his menacing attacks on innocent folk.

“Not if I can keep my head,” Don muttered.

Swinging in his ever-ascending circles, spiraling, reversing to avoid that irksome sameness of turn which might make him dizzy, he kept going higher.

He knew that once he got near the ceiling, that highest point to which an engine can carry an airplane, he would be on equal terms with the Demon, because he could fly past, or execute some other maneuver, by which his propeller blast would upset control of those large top blades, cause the other ship to drop, whereupon, above it, and ever alert to guard against more deadly rockets from the improvised “gun” he saw on the ship’s side, Don could drive down his foe.

To his surprise, before he reached the ceiling, he came level with the other ship.

He saw the pilot, in the moonlight, lift a hand. Instinctively Don prepared to execute some dodging stunt; but all that was released was a white flare.

And in its light Don saw the pilot elevating both hands.

It was the gesture of surrender!

Tamely enough the other allowed his ship to settle. Like a shepherd dog circling a flock, Don went down above the other.

When the swamp was once more close beneath them Don saw that flares were burning, that torches were lighted in various parts of the land beside the Demon’s lair.

Hardly had the pontoons of the helicopter plunged into the water before Don had made his approach, easily guided by the vivid light.

As he swung down, contacting the sheet of water, Don saw, with surprise, that his adversary was no oilskin-cloaked miscreant.

The youth stepping from the helicopter into the electric launch was coppery of skin, black of hair. No other occupied the cockpit.

The launch turned, while Chick and Garry busied themselves with rope, binding the sullen son of Ti-O-Ga, the Indian Garry had met.

“Well,” Vance, the control chief, saluted Don. “You’ve brought down your prisoner. Wish we could say as much.”

“What happened?” Don asked.

As his engine died he listened intently.

“When this Indian ran away from the airport and came here,” Chick explained, “he must have tried to use the helicopter to get away in. But the real Demon jumped out as he got in, letting you go up after the helicopter while he got away.”

“But how could he get away?” Don remonstrated. “There’s only the narrow rim of land, beyond us, on Crab Channel, then another water inlet.”

“Mr. Vance guarded the paths,” Chick admitted, “and Garry picked me up and we beat the grass. I don’t know how he could get away—but he is gone—and with the police to help us beat this part of the swamp—all we’ve found is—just nothing!”

“But he—couldn’t get away!” expostulated Don.

“Couldn’t he?” said Garry, ruefully. “Well, then—where shall we look next?”

But Don did not hit upon the right answer.