"What happened to you?" asked Jack and Dick in chorus.

Swiftly Harry explained. He told of his arrest as a spy and of his escape. And when he mentioned the part that Ernest Graves had played in the affair, Jack and Dick looked at one another.

"We were afraid of something like that," said Jack. "Harry, we've found out a lot of things, and we don't know what they mean! We're sure something dreadful is going to happen to-night. And we're sure, too, that Bray Park is going to be the centre of the trouble."

"Tell me what you know," said Harry, crisply. "Then we'll put two and two together. I say, Jack, we don't want to be seen, you know. Isn't there some side road that doesn't lead anywhere, where I can run in with the car while we talk?"

"Yes. There's a place about a quarter of a mile further on that will do splendidly," he replied.

"All right. Lead the way! Tell me when we come to it. I've just thought of something else I ought never to have forgotten. At least, I thought of it when I took the things out of my pockets while I was changing my clothes."

They soon came to the turning Jack had thought of, and a run of a few hundred yards took them entirely out of sight of the main road, and to a place where they were able to feel fairly sure of not being molested.

Then they exchanged stories. Harry told his first. Then he heard of Dick's escape, and of his meeting with Jack. He nodded at the story they had heard from Gaffer Hodge.

"That accounts for how Graves knew," he said, with much satisfaction. "What happened then?"

When he heard of how they had thought too late of calling Colonel Throckmorton by telephone he sighed.

"If you'd only got that message through before Graves got in his work!" he said. "He'd have had to believe you then, of course. How unlucky!"

"I know," said Jack. "We were frightfully sorry. And then we went out to find where the wire was cut, and they got Dick. But I got away, and I managed to stay fairly close to them. I followed them when they left Dick in a little stone house, as a prisoner, and I heard this—I heard them talking about getting a big supply of petrol. Now what on earth do they want petrol for? They said there would still be plenty left for the automobiles—and then that they wouldn't need the cars any more, anyhow! What on earth do you make of that, Harry?"

"Tell me the rest, then I'll tell you what I think," said Harry. "How did you get Dick out? And did you hear them saying anything that sounded as if it might be useful, Dick?"

"That was fine work!" he said, when he had heard a description of Dick's rescue. "Jack, you seem to be around every time one of us gets into trouble and needs help!"

Then Dick told of the things he had overheard—the mysterious references to Von Wedel and to things that were to be done to the barracks at Ealing and Houndsditch. Harry got out a pencil and paper then, and made a careful note of every name that Dick mentioned. Then he took a paper from his pocket.

"Remember this, Dick?" he asked. "It's the thing I spoke of that I forgot until I came across it in my pocket this morning."

"What is it, Harry?"

"Don't you remember that we watched them heliographing some messages, and put down the Morse signs? Here they are. Now the thing to do is to see if we can't work out the meaning of the code. If it's a code that uses words for phrases we're probably stuck, but I think it's more likely to depend on inversions."

"What do you mean, Harry?" asked Jack. "I'm sorry I don't know anything about codes and ciphers."

"Why, there are two main sorts of codes, Jack, and, of course, thousands of variations of each of those principal kinds. In one kind the idea is to save words—in telegraphing or cabling. So the things that are likely to be said are represented by one word. For instance Coal, in a mining code, might mean 'Struck vein at two hundred feet level.' In the other sort of code, the letters are changed. That is done in all sorts of ways, and there are various tricks. The way to get at nearly all of them is to find out which letter or number or symbol is used most often, and to remember that in an ordinary letter E will appear almost twice as often as any other letter—in English, that is."

"But won't this be in German?"

"Yes. That's just why I wanted those names Dick heard. They are likely to appear in any message that was sent. So, if we can find words that correspond in length to those, we may be able to work it out. Here goes, anyhow!"

For a long time Harry puzzled over the message. He transcribed the Morse symbols first into English letters and found they made a hopeless and confused jumble, as he had expected. The key of the letter E was useless, as he had also expected. But finally, by making himself think in German, he began to see a light ahead. And after an hour's hard work he gave a cry of exultation.

"I believe I've got it!" he cried. "Listen and see if this doesn't sound reasonable!"

"Go ahead!" said Jack and Dick, eagerly.

"Here it is," said Harry. "'Petrol just arranged. Supply on way. Reach Bray Friday. Von Wedel may come. Red light markers arranged. Ealing Houndsditch Buckingham Admiralty War Office. Closing.'"

They stared at him, mystified.

"I suppose it does make sense," said Dick. "But what on earth does it mean, Harry?"

"Oh, can't you see?" cried Harry. "Von Wedel is a commander of some sort—that's plain, isn't it? And he's to carry out a raid, destroying or attacking the places that are mentioned! How can he do that? He can't be a naval commander. He can't be going to lead troops, because we know they can't land. Then how can he get here? And why should he need petrol?"

They stared at him blankly. Then, suddenly, Dick understood.

"He'll come through the air!" he cried.

"Yes, in one of their big Zeppelins!" said Harry. "I suppose she has been cruising off the coast. She's served as a wireless relay station, too. The plant here at Bray Park could reach her, and she could relay the messages on across the North Sea, to Helgoland or Wilhelmshaven. She's waited until everything was ready."

"That's what they mean by the red light markers, then?"

"Yes. They could be on the roofs of houses, and masked, so that they wouldn't be seen except from overhead. They'd be in certain fixed positions, and the men on the Zeppelins would be able to calculate their aim, and drop their bombs so many degrees to the left or the right of the red marking lights."

"But we've got aeroplanes flying about, haven't we?" said Jack. "Wouldn't they see those lights and wonder about them?"

"Yes, if they were showing all the time. But you can depend on it that these Germans have provided for all that. They will have arranged for the Zeppelin to be above the positions, as near as they can guess them, at certain times—and the lights will only be shown at those times, and then only for a few seconds. Even if someone else sees them, you see, there won't be time to do anything."

"You must be right, Harry!" said Jack, nervously. "There's no other way to explain that message. How are we going to stop them?"

"I don't know yet, but we'll have to work out some way of doing it. It would be terrible for us to know what had been planned and still not be able to stop them! I wish I knew where Graves was. I'd like—"

He stopped, thinking hard.

"What good would that do?"

"Oh, I don't want him—not just now. But I don't want him to see me just at present. I want to know where he is so that I can avoid him."

"Suppose I scout into Bray?" suggested Jack. "I can find out something that might be useful, perhaps. If any of them from Bray Park have come into the village to-day I'll hear about it."

"That's a good idea. Suppose you do that, Jack. I don't know just what I'll do yet. But if I go away from here before you come back, Dick will stay. I've got to think—there must be some way to beat them!"