inquired Bob.

Frank laughed in half-embarrassed fashion.

“Oh, one of my hunches,” he said.

His two chums grinned understandingly at each other. It was a recognized fact among them that Frank was super-sensitive and frequently, as a result, received sharp impressions concerning people and events which were unsupported by evidence at the time, but which later proved to be correct. Frank was the slightest of the trio, of only medium height but wiry build, while Bob and Jack both were six feet tall and Bob, besides, had a broad and powerful frame.

“Seeing spooks again?” chaffed Bob.

Immediately, they became more serious as Frank, ignoring the banter, leaned forward and made his proposal:

“That plane landed, and I believe it landed at Starfish Cove. Let’s fly back and take a look. See what’s it like, at any rate.”

“Good idea,” approved Jack.

Bob had been taxying about slowly since landing, in order to keep the engine going and the propeller slowly revolving. Now he picked up speed, straightened 16 out, shifted the lifting plane, and the machine shot forward, skirled over the water and presently took the air.

For some minutes they flew in silence, at no great height, and a little distance out from the coast. Bob’s attention was devoted to the plane, but Frank and Jack scanned the shore with eager eyes. Presently they saw what they were looking for. A strange plane rode in the lazy swell offshore in Starfish Cove. There was nobody aboard. Not a soul was in sight on land. The little stretch of sandy beach, between the two horns of the cove, stretched untenanted back to the thick fringe of trees.

Bob swooped so low the plane almost skimmed the water, and all three obtained a good view of the stranger, before once more Bob soared aloft and forged ahead. Looking back, Frank trained the glasses on the scene. But nobody appeared from among the trees, and, far as they could determine, they were unobserved.

They made a quick run to their own landing field, descended and put the plane away. Not until the doors were closed and locked did they sit down on the skidway outside the hangar to discuss what they had seen. There had been remarks made by all after they had seen the strange plane at close range and on the hasty trip home, but all had been 17 too busy with their own thoughts for extended discussion.

Discovery of the plane had altered their original plans to fly over the secret radio station. They had decided not to advertise their presence as, if Frank was correct in his surmise that the other plane had been watching them, their return would create suspicion and put the mysterious strangers on guard against them.

“They may be on a perfectly legitimate enterprise, whoever they are,” Jack said, as all three took seats on the skidway.

“And we may be fools for butting in where we have no business to be,” said Bob. “That your idea?”

“Yes.”

“But look here,” said Frank. “I have the feeling that there’s something about all this business that isn’t open and aboveboard. I, for one, vote that we do our best to find out what is going on.”

Jack sat silent for several moments.

“That isn’t what concerns me at the present moment, after all,” he said. “Whether these people with their strange plane and their secret radio are on legitimate business or not, doesn’t interest me so much. What puzzles me—and I reckon it puzzles the rest of you, too—is the design of that plane.” 18

The others nodded vigorously.

“What a tiny thing,” was Frank’s comment.

“I was busy and couldn’t see much,” supplemented Bob. “But what impressed me was her short hood. Why, she looked as if she had no engine at all.”

“That’s right,” agreed Frank. “I never saw a plane like it. And I can’t recall any designs of that nature, either. It must be a foreign-built plane, one of those little one-man things the Germans and French have been building.”

Jack shook his head, puzzled.

“There’s something strange about it,” he said, “a little thing like that, with practically no engine space. Another thing that you fellows want to remember, too, is that probably it has been flying about here for some time, yet we have never heard it. Now, down here the sound of most planes would travel far, in this quiet and secluded place, where there are no competing noises.”

“Why do you say it has been flying about here for some time?” asked Bob.

“Well, the familiarity with which the aviator landed shows he’s been at Starfish Cove before. Evidently, after landing he struck inland to that secret radio station, because we saw no sign of him.”

“We haven’t been up in the air for three weeks,” said Frank. “That plane might easily have come 19 and gone in that time without our seeing it. But, surely, as Jack says, we would have heard it at some time or other. Haven’t either of you heard the sound of a plane lately?” he appealed to the others. “I know I haven’t.”

Bob and Jack both shook their heads in negation.

“No planes ever come out this way,” Bob said. “They fly south or north of us, but not out here. I haven’t heard anything.”

Jack rose and stretched.

“Well, I, for one, vote that we do not pursue our investigations into this mystery by going back and, perhaps, getting peppered with gunshot.”

“But, Jack,” protested Bob, the impetuous, “we want to know what’s going on. You can’t have a mystery dumped right in your own dooryard without digging into it.”

Frank was thoughtful.

“That’s true, Bob, old thing,” he said. “Just the same, I agree with Jack. What do you say to laying the matter before Uncle George and Mr. Hampton at dinner? Jack and his father are coming over to our house to-night, you know.”

“Good,” said Jack. “We can put it up to them, and, perhaps, they will know something about the man who owns that land around Starfish Cove, where this secret radio is located.” 20

Big Bob grumbled. Delay irked his soul.

“All right, you old grumbler,” laughed Frank. “Come on, I’ll give you some action. We have several hours of good daylight left before dinnertime. I’ll take you on at tennis. Della and I will play you and Jack, and we won’t give you time to worry about anything.”

Della was Bob’s sister, two years younger than he. Frank, whose parents were dead and who lived with the Temples, referring to Mr. Temple, his guardian, as “Uncle George,” was very fond of her. The others joshed him about Della frequently. Bob took occasion to do so now, as the three walked away from the hangar toward the Temple home and tennis courts.

“Huh,” he said, “you’ll be looking at your partner so often you won’t be able to play. Why, you won’t even be good practice for Jack and me.”

Della was lithe-limbed, quick of eye and strong of wrist, a born tennis player. As for Frank, tennis was the one sport at which he could excel his chums. The result was that, despite the strong game played by Jack and Bob, Frank and Della won two sets, 7-5, 8-6.

Mr. Hampton appeared on the scene when the second set stood at six-all, bringing with him an alert, thin-faced man of middle age, clad in the uniform of a colonel in the United States Engineers. Mr. Temple with his wife emerged from the house to greet their guests, and all four were interested spectators of the two concluding games which