“Well,” replied Jack, “I told him we had learned very little; but that the secret of the whole affair was at the mouth of the Thames; that that was where we were heading for when we were captured.”

“And did you give him some kind of a hint as to the nature of the mystery?”

Jack smiled.

“Well, yes,” he said.  “I told him he would find some queer explosives there and a large number of swift torpedo boat destroyers, equipped with submarine nets.  I told him that these made nightly raids into the Channel and the North Sea and thus disposed of the German submarines.”

“And he believed it?”

“He seemed to.  But that was the best I could do on short notice and knowing no more than I do of the matter.”

“Well, it wasn’t so bad,” grinned Frank.

“I flatter myself that it could have been worse,” returned Jack modestly.

They fell into a long silence, which was suddenly broken by Frank, who exclaimed anxiously: 

“Say!  Do you suppose that could have been a trap?”

“Trap?” repeated Jack.  “What do you mean?”

“I mean, do you think Lord Hastings or some of the other British officers sent that man here simply to get evidence against us?”

“By Jove!” exclaimed Jack.  “I never thought of that.  Whew!  Maybe I have gotten us into worse trouble than ever.”

“If it was a trap, I guess you have,” returned Frank glumly.  “Yes, it was a good job you did, I don’t think.”

“Wait a minute, though,” said Jack.  “If it were a trap, it would be plainly evident we didn’t know what we were talking about.  That should be enough to let us out.”

“Unless they believe we smelled a mouse and acted accordingly,” returned Frank.

“Oh, well,” said Jack, “there is no use worrying about it.  We’ll have to take what comes and that’s all there is about it.  Now, I’m going to rest a bit.  When they come in, wake me up.”

He leaned back in his chair and closed his eyes; and in the meantime Frank continued to turn events over in his mind.

Half an hour later Lord Hastings, the man who had led the party that captured the boys, a figure attired in the uniform of a colonel of infantry, and several others entered the room.  The uniformed man Frank immediately took to be Colonel Masterson, and he was right.  Frank nudged Jack and the latter opened his eyes.  Then both boys got to their feet.

“So these are the German spies, eh?” said Colonel Masterson.

“That’s not true,” declared Frank hotly.  “We are no more spies than you are.”

“So?” said the colonel insinuatingly.  “Then how does it happen you hold such friendly conversation with another man who is under suspicion?”

Frank’s heart sank.  So it had been a trap, after all.

“I can explain that, sir,” said the boy, turning to Lord Hastings.

“I have no doubt of that,” was the reply.  “The question is, can you explain it satisfactorily?”