sufficient sunlight.

Tom and his father started their planting. But no sooner had the first
plants been embedded than fish darted in to nibble them. Even the roots
disappeared into their greedy maws.

[Illustration]

"Looks as though we'll have to build some sort of net enclosure around
and over our farm," Mr. Swift said, after they had climbed back into
the boat. "But at least your hydrolung device is a great success, son!"

Tom was thoughtful. "Dad, I wonder if the fish would eat those plants
from space which you've been growing under salt water?"

Tom was referring to certain strange plants rocketed to earth by unknown
space friends with whom the Swifts had been in communication.

"I have a hunch," Tom went on, "that the fish might be repelled by the
unusual scent of those space plants. If so, we could scatter them among
the earth plants to keep the fish away."

Mr. Swift was impressed by Tom's idea. As soon as they had returned to
Enterprises, he proposed that the experiment get under way.

Tom volunteered to undertake the job at once with Bud. While the young
inventor phoned his copilot, Mr. Swift went to his own laboratory to
prepare the plants for shipment.

Twenty minutes later the boys took off in a jet. The plants had been
parceled in transparent plastic film. Glistening with a red metallic
sheen, they looked somewhat like tulips with honeycombed centers.

"Scarecrow plants to drive off fishes," Bud joked. "What will scientists
think of next!"

Tom laughed, then abruptly frowned. "Hey! What's that character up to?"
he said. "Trying to buzz us?"

A sleek gray jet without markings was arrowing in on them from three
o'clock. Bud flicked on the radio and barked a warning. The plane made
no response. As it kept coming, Tom increased speed--then rolled, dived,
and changed course, but failed to shake off their pursuer.

Bud, meanwhile, was frantically calling Enterprises and a nearby
airport, but getting no response. Yet their radio was working, for a
voice suddenly crackled:

"_Follow the mystery plane for a landing and you won't be harmed!_"

CHAPTER V

A HUNCH PAYS OFF

Dismayed, Tom and Bud stared at each other. Apparently the enemy ship
had blanked out their radio communication to all points except the
mystery plane.

"Who are you and what do you want?" Tom said into his microphone.

The voice replied crisply, "_You'll find out when the time comes!_"

Tom flicked off his mike an