Bud shrugged. "Let's find out." He ordered a change of course, hard to the right, and gunned the jets to bring the jetmarine directly on the mystery object's trail.

"It's a sub, all right," he said a short time later, listening again over the hydrophones.

"Pretty close to Fearing Island, isn't it?" put in Mel Flagler. "That's a government-restricted area."

Bud nodded grimly. "But staying just out of sonar range from the base."

The jetmarine closed steadily on its quarry. In a few minutes they were able to make it out dimly through the cabin window, dead ahead.

"That's sure no U.S. Navy sub that I know of," Bud said. "Probably an enemy snooper."

"What if they spot us?" Zimby asked.

Bud chuckled. "That's the beauty of it, pal! Don't forget. With this new antidetection gear we're invisible to them. At least as long as they don't run into us or we into them," he added.

"Or unless they have superdetection equipment we don't know about,"

cautioned Mel Flagler.

As they talked, the unidentified submarine was bearing steadily toward the mainland. Fathometer soundings showed it was on a steep upward slope of the continental shelf.

Presently a foaming gush of bubbles showed that the sub ahead was blowing its tanks. The jetmarine followed as it surfaced and Bud hastily manned the periscope.

"What're they up to?" Mel asked tensely.

"Don't know yet, but the hatch is opening," Bud reported. Suddenly he gave an excited gasp. "Jumpin' jets! They're sending out a couple of frogmen!"

Bud's companions were electrified by the news.

"Spies!" Zimby exclaimed.

"What do we do now?" piped up Mack Avery, the third man in Bud's crew.

"Hadn't we better radio the Coast Guard and the FBI?"

Bud wrenched away from the eyepiece. "I have another idea! Any of you fellows game to go with me and capture those spies?"

All three of his companions volunteered eagerly. Bud chose Mel Flagler, then took another sight through the periscope.

"The sub's submerging again," he reported. "That'll give us a clear field. Zimby, you and Mack keep an eye on that baby while we're gone, and be plenty careful she doesn't spot you!"

"Roger! And take this roll of wire to tie up your prisoners."

Hastily Bud and Mel changed into swimming trunks and donned hydrolungs.

They went out through the air lock, plunged into the bracing salt water, and switched on their ion-drive units.

"Can you see 'em?" Mel asked over his mike.

"Not yet. Let's speed up before we lose 'em completely!"

Both pushed their ion drives to capacity, scanning the water ahead in all directions.

"There they are!" Bud exclaimed presently. He pointed to two tiny figures, barely visible in the distance.

"Wow! They're sure not wasting any time!" Mel muttered. "Let's step on it, Bud! They'll be ash.o.r.e in a minute!"

A darting school of sea ba.s.s screened the figures briefly from view. As the fish flickered past, Mel and Bud saw the frogmen breast-stroke up toward the surface and break water.

Bud and Mel followed. Ahead lay a barren stretch of beach, humped with sand dunes. It was skirted beyond by a thick fringe of trees.

"They certainly picked a perfect spot for a sneak landing!" Bud thought.

The beach seemed totally deserted, with no sign of human habitation.

By this time, the frogmen were scrambling ash.o.r.e. Within moments, Bud and Mel were on their heels. The raiders whirled in dismay as they caught the sound of footsteps rushing up behind them through the sand.

Bud and Mel hurled themselves forward, each dropping a man with a flying tackle. All four went down in a struggling, kicking tangle of arms and legs.

The battle was rough but short. Bud and Mel had the advantage of surprise, and soon pommeled and grappled their foes into submission.

Bud, astride his opponent's chest with knees pinning the man's arms, unlooped from his belt the wire he had brought.

"Here! Take some of this and wire your man's wrists together!" Bud told Mel.

When the frogmen were safely bound, Bud and Mel allowed them to stand up. Neither captive tried to escape.

"Now, my sneaky friends, talk!" Bud snapped. "What kind of a sightseeing trip did you plan?"

The frogmen's jaws remained tightly clamped. Both looked flushed and sullen as they faced their captors.

"Got their lips zipped, I guess," Mel said disgustedly.

Bud decided to try another tack. "Doesn't matter," he said carelessly.

"We know they're pals of the Mirovs."

Both men started as if they had been stung. Bud followed up quickly, hoping to prod them into some unguarded remark.

"Just as we thought!" he snarled. "A couple of low-down Brungarian rebels! And up to their usual amateurish spy stunts!"

The raiders' eyes blazed, but they maintained silence. Both, however, kept darting looks of keen interest at the Americans' hydrolung gear.

Just as Bud was wondering how he could get the prisoners to the nearest police headquarters, a jeep came bouncing into view across the sand.

"Hey! Police!" Mel exclaimed with a happy grin.

"We're in luck," Bud said. "They can take these creeps off our hands."

The jeep braked to a halt a few yards away, and two uniformed officers hopped out.

"What's going on here?" said one, who was wearing a sergeant's stripes.

The jeep had the words BEACH PATROL stenciled on it in white paint.