"Battery or mains power. I believe the Gestapo's favorite trick, when they're trying to locate the source of an illicit radio transmission, is to switch off the town's electricity block by block until the broadcast is cut off"

Dieter nodded.

"Well, with this set, if you lose the house current, you just have to reverse this plug, and it switches to battery operation."

"Very good." Dieter would pass that on to the Gestapo, in case they did not already know.

Helicopter plugged the power lead into an electrical outlet, then took the aerial wire and asked Stephanie to drape it over a tall cupboard. Dieter looked in the kitchen drawers and found a pencil and a scratch pad that Mademoiselle Lemas had probably used to make shopping lists. "You can use this to encode your message," he said helpfully.

"First I'd better figure out what to say." Helicopter scratched his head, then began to write in English:

ARRIVED OK STOP CRYPT RENDEZVOUS

UNSAFE STOP NABBED BY GESTAPO BUT

GOT AWAY OVER

"I suppose that's it for now," he said.

Dieter said, "We should give them a new rendezvous for future incomers. Say the Caf de La Gare next to the railway station."

Helicopter wrote it down.

He took from the case a silk handkerchief printed with a complex table showing letters in pairs. He also took out a pad of a dozen or so sheets of paper printed with five-letter nonsense words. Dieter recognized the makings of a one-time-pad encryption system. It was unbreakable-unless you had the pad.

Over the words of his message, Helicopter wrote the five-letter groups from the pad; then he used the letters he had written to select transpositions from the silk handkerchief Over the first five letters of ARRIVED he had written the first group from his one-time pad, which was

BGKRU. The first letter, B, told him which column to use from the grid on the silk handkerchief At the top of column B were the letters Ae. That told him to replace the A of ARRIVED with the letter e.

The code could not be broken in the usual way, because the next A would be represented not by e but by some other letter. In fact, any letter could stand for any other letter, and the only way to decrypt the message was by using the pad with the five-letter groups. Even if the code breakers could get hold of a coded message and its plain-language original, they could not use them to read another message, because the next message would be encoded with a different sheet from the pad- which was why it was called a "one-time" pad. Each sheet was used once, then burned.

When he had encrypted his message, Helicopter flicked the on/off switch and turned a knob marked in English "Crystal Selector." Looking carefully, Dieter saw that the dial bore three faint markings in yellow wax crayon. Helicopter had mistrusted his memory and had marked his broadcast positions. The crystal he was using would be reserved for emergencies. Of the other two, one would be for transmission and the other for reception.

Finally he tuned in, and Dieter saw that the frequency dial was also marked with yellow crayon.

Before sending his message, he checked in with the receiving station by sending:

HLCP DXDX QTC1 QRK? K

Dieter frowned, figuring. The first group had to be the call sign "Helicopter." The next one, "DXDX," was a mystery. The number one at the end of "QTC1" suggested that this group meant something like: "I have one message to send you." The question mark at the end of "QRK?" made him think this asked if he was being received loud and clear. "K" meant "Over," he knew. That left the mysterious "DXDX."

He tried a guess. "Don't forget your security tag," he said.

"I haven't," Helicopter said.

That must be "DXDX," Dieter concluded.

Helicopter turned to "receive" and they all heard the Morse reply:

HLCP QRK QRV K

Once again, the first group was Helicopter's call sign. The second group, "QRK," had appeared in the original message. Without the question mark, it presumably meant "I am receiving you loud and clear." He was not sure about "QRV," but he guessed it must mean "Go ahead."

As Helicopter tapped out his message in Morse, Dieter watched, feeling elated. This was the spy catcher's dream: he had an agent in his hands and the agent did not know he had been captured.

When the message was sent, Helicopter shut down the radio quickly. Because the Gestapo used radio direction-finding equipment to track down spies, it was dangerous to operate a set for more than a few minutes.

In England, the message had to be transcribed, decoded, and passed to Helicopter's controller, who might have to consult with others before replying; all of which could take several hours, so Helicopter would wait until the appointed hour for a response.

Now Dieter had to separate him from the wireless set and, more importantly, from his coding materials. "I presume you want to contact the Bollinger circuit now," he said.

"Yes. London needs to know how much of it is left."

"We'll put you in touch with Monet, that's the code name of the leader." He looked at his wristwatch and suffered a moment of sheer panic: it was a standard issue German Army officer's watch, and if Helicopter recognized it the game would be up. Trying to keep the tremor out of his voice, Dieter said, "We've got time, I'll drive you to his house."

"Is it far?" Helicopter said eagerly.

"Center of town."

Monet, whose real name was Michel Clairet, would not be at home. He was no longer using the house; Dieter had checked. The neighbors claimed to have no idea where he was. Dieter was not surprised. Monet had guessed that his name and address would be given away by one of his comrades under interrogation, and he had gone into hiding.

Helicopter began to close up the radio. Dieter said, "Does that battery need recharging from time to time?"

"Yes-in fact they tell us to plug it in at every opportunity, so that it's always fully charged."

"So why don't you leave it where it is for now? We can come back for it later, by which time it will be charged. If anyone should come in the meantime, Bourgeoise can hide it away in a few seconds."

"Good idea."

"Then let's go." Dieter led the way to the garage and backed the Simca Cinq out. Then he said, "Wait here a minute, I have to tell Bourgeoise something."

He went back into the house. Stephanie was in the kitchen, staring at the suitcase radio on the kitchen table. Dieter took the one-time pad and the silk handkerchief from the accessories compartment. "How long will it take you to copy these?" he said.

She made a face. "All those gibberish letters? At least an hour."

"Do it as fast as you can, but don't make any mistakes. I'll keep him out for an hour and a half"

He returned to the car and drove Helicopter into the city center.

Michel Clairet's home was a small, elegant town house near the cathedral. Dieter waited in the car while Helicopter went to the door. After a few minutes, the agent came back and said, "No answer."

"You can try again in the morning," Dieter said. "Meanwhile, I know a bar used by the Resistance." He knew no such thing. "Let's go there and see if I recognize anyone."

He parked near the station and picked a bar at random. The two of them sat drinking watery beer for an hour, then returned to the rue du Bois.

When they entered the kitchen, Stephanie gave Dieter a slight nod. He took it to mean she had succeeded in copying everything. "Now," Dieter said to Helicopter, "you'd probably like a bath, having spent a night in the open. And you certainly should shave. I'll show you your room, and Bourgeoise will run your bath."

"How kind you are."