“You mean that Mr. Hoff goes to a different bookstore each day to leave a code message?”

“That’s it.  The spy who gets the messages each morning calls him up by ’phone, mentioning just the one word.  From that Mr. Hoff knows just where to go, concealing the message in a book before agreed upon.”

“The fifth book,” interrupted Dean.

“Not always,” explained Fleck.  “It depends on whether there are five letters in the name telephoned.  I have located and copied several more of the messages.”

“But who gets the messages he leaves?  Who takes them away from the bookshops?” asked Jane, mindful of her own failure in that respect.

“It’s a girl, or rather two girls together, though possibly only one of them is in the plot.  Very likely the other may not know what her companion is doing.”

“To whom does this girl take them?”

“That is still a mystery,” said the chief.  “We have ascertained who the girl is, where she lives.  Her actions have been watched and recorded for every hour in the twenty-four for the last three days, and yet we don’t know what she does with these messages.  Carter has a theory—­tell us about it, Carter.”

“In accordance with instructions,” began Carter, as if he was making out a report, “I had operatives K-24 and K-11 shadow the party suspected.  On two different occasions they followed her to a bookstore and back home again.  She was accompanied on one occasion by her younger sister.  Each time she went directly home and stopped there, neither she nor her sister coming out again, and no person visiting the apartment, but—­”

“Here’s the interesting part,” interrupted Fleck.

“On both occasions within a couple of blocks of the bookstore she passed a man with a dachshund.  She did not speak to the man, but each time she stopped to pet the dog.”

“Was it the same man both times?” asked Dean.

“Apparently not,” replied Carter, “but it may have been the same dog.  Dachshunds all look alike.”

“Go on,” said the chief.

“Now my theory is that that girl was instructed to walk north until she met the man with the dog.  I’ll bet anything that code message went under the dog’s collar.  The next time she gets a message I’m going to get that dog.”

“It seems preposterous,” scoffed Dean.

“Rather it shows,” said Fleck, “that these spies all suspect they are being watched, and that they resort to the most extraordinary methods of communication to throw off shadowers.  They have used dachshunds before.  There’s a New England munition plant to which they used to send a messenger each week to learn how their plans for strikes and destruction were progressing.  They put a different man on the job each time to avoid stirring up suspicion.  At the station there would always be two children playing with a dachshund.  The spy would simply follow them as if casually, and they would lead him to a rendezvous with the local plotters.  Now, Miss Strong,” he said, turning to Jane, “I brought you down here for two reasons.  First, to give you an inkling of how important your task is, and second, to ask you to undertake still another task for us.  Are you still willing to help?”