"But I don't understand," the housekeeper said. "A man came with a note from you, Tom, and in it you said to have him take the tank, with Koku and the men who know how to run it. We were so glad to hear from you, and know that you were all right, that we didn't think of anything else, your father and I. So he went out and saw that the tank got off all right. Koku was glad, for it's the first chance he'd had to ride in it." "Who was the man who brought the note?" asked Tom, and he was striving to be calm. "To think of poor old dad playing right into the hands of the plotters!" he added, in an aside to Ned. "Well, I don't know who the man was," said Mrs. Baggert. "He seemed all right, and of course having a note from you--" "Who has that note now?" asked Tom quickly. "Your father." "Come on," and Tom led the way back to the house. "I'll have a look at that document, which of course I never wrote, and then we'll get after the plotters and the tank." "She ought to be easy to trace," observed Mr. Damon. "Bless my fountain pen, but she ought to be easy to trace! She will leave a track like a giant boa constrictor crawling along." "Yes, I guess we can trace her, all right," assented Tom Swift; "but the point is, will there be anything left of her? That's what I'm afraid of now." Mr. Swift was still excited, but his worry had subsided as soon as he knew Tom was safe. "The whole thing is a forgery, but fairly well done," Tom said, as he looked at the paper his father gave him--a brief note stating that Tom was well, but detained on business, and that the tank was to be brought to him, just where the bearer of the note would indicate. Koku, the giant, and several of the machinists, who knew how to operate the big machine, were to go with it, the note said. "That made me sure everything was all right," said Mr. Swift. "I knew, of course, Tom, that plotters might try to get hold of your war secret, but I didn't see how they could if Koku and some of your own men were in possession." "They couldn't--as long as they remained in possession," Tom said. "But that's the trouble. I'm afraid they haven't. What has probably happened is that under the direction of this man, who brought the forged note from me, Koku and the others took the tank where he directed them, thinking to meet me. Then, reaching the place where the rest of the plotters were concealed, they overpowered Koku and the others and took possession of the machine." "They'd have trouble with Koku," suggested Ned. "Yes, but even a giant can't fight too big a crowd, especially if he is taken by surprise, and that's probably what happened," remarked Tom. "Now the question is where is the tank, and how can we get her back? Every minute counts. If those German spies and their helpers remain in possession long, they'll find out enough of my secrets to enable them to duplicate the machine, and especially some of the most exclusive features. We've got to get after 'em!"