"Captain Smith" had leaped to his feet, quivering with anger. But it was too late. His cellmate, by answering to the name of "Mirov," had given away their nationality!  
Tom and Ames exchanged grins of triumph.
 
"No doubt you recall what happened to Streffan Mirov," Tom went on, pressing his advantage. "Or should I say the late Streffan Mirov? Our last report was that he had been tried and condemned by your own government. Perhaps you can give us news of his fate?"
 
The wavy-haired prisoner's eyes blazed with hate. "Grin while you can, Tom Swift! Because of you, my brother Streffan is now serving a long prison sentence! But I, Dimitri Mirov, will get revenge!"
 
"You blame Tom Swift because your brother botched his job of claiming the satellite Nestria by force and fraud?" Ames taunted.
 
"Our space friends moved that asteroid into orbit around the earth," Tom added. "We claimed it by right of first landing. Even your own leaders couldn't agree to Streffan's crazy scheme to destroy everything."
 
Dimitri Mirov lost all control and burst into a volley of guttural Brungarian abuse.
 
"I warn you, Swift!" he choked. "Jailing us will not make you safe—or your projects, either!"
 
A blow to the head from "Captain Smith" sent Mirov reeling back against the wall. "Fool! Maybe that will quiet you!" the pilot snapped viciously. "You have said too much already!"
 
"Let's go, Tom," said Ames. "We've learned the information we came for."
 
The prisoners could only glare in baffled rage through the cell bars as Tom and the security chief turned their backs and walked away.
 
"Nice going, Tom," Ames murmured. "Your hunch certainly paid off." Chief Slater added his congratulations when he heard how Tom had trapped Mirov into disclosing his identity.
 
Both Tom and Ames were grave as they drove back to the plant. Neither took Mirov's threats lightly.
 
Tom pondered another angle. Were the Brungarian rebels perhaps responsible for the attempted theft of the Jupiter-circling missile?
 
Ames was inclined to think so. "Moreover," he forecast, "it's a cinch they haven't thrown their last punch. I'll pass the word to the FBI and Central Intelligence."
 
After lunch Tom flew to Fearing Island with Bud, eager to tackle their interrupted job of rooting the space plants into the undersea silt beds. Zimby Cox, a sandy-haired, freckle-faced jetmariner, volunteered to pilot a motor launch for them.
 
They sped across the water, then dropped anchor at the farm site. Tom and Bud donned their hydrolung gear and went over the side, each clutching containers of the space plants.
 
Reaching bottom, they glided about in the shadowy green water, embedding the plants at far-spaced intervals. The Tomasite-producing plants had been almost completely devoured. A few fish were darting about, but they swam off quickly at the boys' approach. To Tom's delight, they showed no sign of returning.
 
"Looks as if our keep-off signs are working," Tom said with a pleased chuckle when the boys finally surfaced and climbed back aboard the boat.
 
Bud nodded. "Smart idea, all right." Then he scowled thoughtfully. "But if you ask me, skipper, fishes aren't the only thieves you'll have to guard against."
 
"Meaning?"
 
"Mirov's pals," Bud replied. "If it's the space plants they were after when they pulled that aerial hijack attempt, they could take them easily from these silt beds."
 
Tom sobered. "You have a point there. I'd better have an audio screen set up around this whole area. That'll act as a burglar alarm—and help discourage the fish, too."
 
Twenty minutes later the boys were winging back to the mainland. When Tom reached his office, he called in Gib Brownell, an Enterprises engineer.
 
"Got a job for me, skipper?"
 
Tom handed him a hastily scribbled diagram of the audio-screen setup. "One of those hurry-up deals, Gib," he said with an apologetic grin. Tom explained his plan. "We'll use transmitter buoys, monitored by an alarm system at base headquarters on Fearing."
 
Brownell studied the diagram and nodded. "Right. We can have it set up in twenty-four hours."
 
As Brownell left the office, the telephone jangled. Tom reached for it.
 
"Admiral Walter calling." His voice was tense. "Important news, Tom. One of our subs has picked up a clue that someone has been operating in the missile search area."
 
"What sort of clue, sir?" Tom asked.
 
"A compressed-air caisson for underwater work. It had been driven into the silt and then abandoned." Admiral Walter added that photographs and a section of the caisson were being flown to the Naval Research Laboratory for careful study. "I'll have a full report transmitted to you by video as soon as it reaches my desk."
 
Tom thanked the admiral and hung up, feeling more uneasy than ever. The report came through the following morning. Tom absorbed the contents, then gave a low whistle.
 
"Trouble?" asked Bud, who had just dropped into the office with some flight-test data on a new Swift superjet.
 
"Our old enemies again." Tom shoved the papers across his desk.
 
The report stated that both the design and manufacturing techniques used in making the caisson indicated that it was of Brungarian origin. A spectrographic analysis of the steels confirmed the theory. Their metallurgical content agreed with known Brungarian steel formulas.
 
"The sneaky rats!" Bud cried out. "Well, at least we know now who sabotaged our missile recovery."
 
As Tom paced about the office, Bud added, "What do you suppose they were using the caisson for?"
 
"Probably as a base for some heavy, rotating search equipment," the young inventor surmised.
 
"But why ditch it?"
 
Tom shrugged. "An optimistic guess is that they spotted our Navy search force and pulled out quickly, fearing a surprise attack."
 
"What's a pessimistic explanation?" Bud asked.
 
"Mission completed," Tom said grimly. "No need for them to stick around if they'd already snagged the missile."
 
Bud scowled at the thought. "Oh, no! That mustn't be true!"
 
Tom plopped down at his desk, frowning. "Bud, I've been itching to get to work on a non-detectable sub, like the one that attacked us. But maybe it would be smarter to get a line on Mirov's pals first."
 
"You mean down in the South Atlantic?"
 
Tom nodded. "I'd sure like to know if they found that missile."
 
"You and I both, pal!" Bud agreed. "Hey! We could use the electronic hydrolungs for scouting around!" he added eagerly.
 
"I intend to," Tom said. "But we'll need speed to cover the area. So first I want to add an ion drive to our equipment."
 
"Ion drive? For underwater?" Bud, who was familiar with ion propulsion for spaceships, wrinkled his brow in a puzzled frown.
 
"A goofy idea just occurred to me, but I think it may work out," Tom replied. He seized a pencil and began explaining what he had in mind.
 
The drive unit would take water into itself, separate the ionized molecules, and expose them to an electric field. Thus a stream of water would be forced out. This procedure, in turn, would set up a siphoning action through a central tube—in effect, creating a small but powerful water-jet motor.
 
"We'll be human submarines!" Bud exclaimed.
 
By the time Bud left the laboratory half an hour later, Tom had already plunged into work on his newest invention. The idea was simple enough in itself, Tom felt. The main problem would be the design job—laying out a compact, lightweight unit which a swimmer could easily carry on his back.
 
Fascinated, the young inventor worked late into the evening, stopping only in response to a telephone plea from Mrs. Swift. By midmorning the next day, Tom had assembled a pilot model of his ion-drive jet. In appearance, it was a slender metal cylinder, two feet long, with an inner concentric tube projecting at each end.
 
Tom had ordered a tank set up in his laboratory to test the unit. The tank was filled chest-deep with water, and the ion drive was mounted on a unitrack running the length of it. Tom set up his control board alongside, with the main power switch within easy reach. The drive unit was connected to the board by a suspended cable.
 
"Boy, this'll be like playing with a speedboat in a bathtub!" Tom thought with a chuckle as he changed into swim trunks.
 
He climbed into the tank and slid the drive unit to one end of its track. Then Tom metered out power slowly. With a gentle whoosh, the ion-drive unit whizzed along the unitrack to the other end of the tank.
 
"Not bad," Tom muttered, a pleased grin on his face. "Now I'll rev it up a little."
 
He slid the drive unit back to starting position, then opened the switch wider. He had just started across the tank himself when suddenly he became powerless to move.
 
Tom was pinned helplessly against the wall of the tank by the powerful water-jet exhaust! And the control switch was beyond his reach!