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."