The boys exchanged looks of fear through their face masks as the knifelike hull and conning tower of the submarine loomed gray and ghostly.  
Was the sub Brungarian? And what was it up to? Were the two young skin divers about to be run down or kidnaped?
 
Or was its crew friendly?
 
"Better not chance it," Tom decided fast. He caught Bud's eye again and motioned upward with a jerk of his thumb. "Topside, pal!"
 
"Roger!" Bud's lips shaped the word silently behind his face mask.
 
In a twinkling both boys flicked their density controls and zoomed upward. The sub at once seemed to betray a hostile intent. It blew its tanks and planed upward in pursuit. But Tom and Bud easily pulled away. Their density units worked like magic, shooting them straight toward the surface.
 
"Wow!" Bud shoved back his face mask as they broke water. "That baby was after us and no mistake!"
 
Tom nodded, treading water. "Let's not stick around here, either! We'll soon have company again if we do!"
 
Bud did not argue. "Where to, skipper?"
 
In the fresh salt air, with the sunshine sparkling on the waves, it was hard to believe that an enemy submarine was hot on their trail. But both youths realized their peril was growing by the moment.
 
"Back toward the Sea Hound," Tom said, pointing north-northwest. "Submerge as we go!"
 
Bud circled his thumb and forefinger, then adjusted his mask, and the two boys plunged back in. On a sloping downward course, they sped along like undersea rockets, their ion jets functioning perfectly. Minutes later, they sighted the seacopter.
 
Hank waved to them through the cabin window as they glided past. The air lock opened speedily and the two boys entered. Both heaved sighs of relief when they were safely inside.
 
"Somethin' wrong?" Chow asked, sniffing trouble.
 
"A strange submarine," Tom reported. "Brungarian more than likely. It may be heading this way if they've tracked us."
 
"A sub?" Hank was startled. "We've picked up nothing on sonar!"
 
"Check again," Tom ordered.
 
The sonarman bent to his scope and Hank listened intently over the hydrophones. Neither could detect any sign of another craft.
 
"Probably the same one that fired on us the last time," Tom said grimly. "We'd better clear out before they take another pot shot at us."
 
Hank sent the Sea Hound zooming toward the surface while the boys changed quickly into slacks and T shirts. Then Tom took over the controls for the flight home.
 
"Brand my vitamin vittles! Are we just goin' to turn tail an' run every time them varmints come skulkin' around?" Chow fumed as the seacopter arrowed northward.
 
"Not if I can help it," Tom vowed. "But first I must figure out a way to make our own craft invisible, so to speak. It's the only way to protect our American crews, Chow, if we hope to do any secret digging for that lost missile."
 
"Want another suggestion, skipper?" Bud put in. "This one is about the hydrolung."
 
"Sure. Speak up."
 
"How about putting some sort of communications system into our hydrolung gear? If I hadn't been close enough to grab you when I spotted that sub, it might have been curtains, pal!"
 
"You're right," Tom agreed. "I'll get to work on it."
 
It was sunset when Fearing Island came into sight. The boys flew a Pigeon Special back to Enterprises, where Tom phoned a full report on the mystery sub to the Navy Department. Then the two chums drove to the Swift home for a late supper.
 
Phyl Newton was visiting Sandy that evening, but the girls displayed a marked coolness toward Tom and Bud. Instead of engaging in conversation, they retired to Sandy's room upstairs to play records, while Mrs. Swift served the boys a warmed-up but tasty meal of roast beef and mince pie.
 
"What's wrong? Are we repulsive or something?" Bud asked as they ate.
 
Tom shrugged, concentrating on a mouthful of roast beef. "Search me. We sure don't seem very popular with the girls tonight."
 
Mrs. Swift, overhearing their remarks in the kitchen, smiled but maintained a diplomatic silence.
 
Suddenly Bud slapped his forehead. "Good night! No wonder!"
 
Tom looked up with a grin of interest. "Well, what have we done?"
 
"It's what we haven't done, pal!" Bud retorted. "We had a date this afternoon, remember? That beach party and dance put on by Sandy and Phyl's school sorority!"
 
Tom gulped. "Oops! Boy, we really did pull a boner this time! I completely forgot!"
 
As they finished supper, the boys discussed various ways to make amen............