WIRELESS OPERATORS WERE not quite invisible.

They lived in a spirit world where their ghostly shapes could be dimly seen.

Peering into the gloom, searching for them, were the men of the Gestapo's radio detection team, housed in a cavernous, darkened hall in Paris.

Dieter had visited the place.

Three hundred round oscilloscope screens flickered with a greenish light.

Radio broadcasts appeared as vertical lines on the monitors, the position of the line showing the frequency of the transmission, the height indicating the strength of the signal.

The screens were tended, day and night, by silent, watchful operators, who made him think of angels observing the sins of humankind.

The operators knew the regular stations, either German-controlled or foreign-based, and were able to spot a rogue instantly.

As soon as this happened, the operator would pick up a telephone at his desk and call three tracking stations: two in southern Germany, at Augsburg and Nuremberg, and one in Brittany, at Brest.

He would give them the frequency of the rogue broadcast.

The tracking stations were equipped with goniometers, apparatus for measuring angles, and each could say within seconds which direction the broadcast was coming from.

They would send this information back to Paris, where the operator would draw three lines on a huge wall map.

The lines intersected where the suspect radio was located.

The operator then telephoned the Gestapo office nearest to the location.

The local Gestapo had cars waiting in readiness, equipped with their own detection apparatus.

Dieter was now sitting in such a car, a long black Citroensparked on the outskirts of Reims.

With him were three Gestapo men experienced in wireless detection.

Tonight the help of the Paris center was not required: Dieter already knew the frequency Helicopter would use, and he assumed Helicopter would broadcast from somewhere in the city (because it was too difficult for a wireless operator to lose himself in the countryside).

The car's receiver was tuned to Helicopter's frequency.

It measured the strength, as well as the direction, of the broadcast, and Dieter would know he was getting nearer to the transmitter when the needle rose on the dial.

In addition, the Gestapo man sitting next to Dieter wore a receiver and an aerial concealed beneath his raincoat.

On his wrist was a meter like a watch that showed the strength of the signal.

When the search narrowed down to a particular street, city block, or building, the walker would take over.

The Gestapo man in the front seat held on his lap a sledgehammer, for breaking doors down.

Dieter had been hunting once.

He did not much like country pursuits, preferring the more refined pleasures of city life, but he was a good shot.

Now he was reminded of that, as he waited for Helicopter to begin sending his coded report home to England.

This was like lying in the hide in the early dawn, tense with anticipation, impatient for the deer to start moving, savoring the thrill of anticipation.

The Resistance were not deer but foxes, Dieter thought, skulking in their holes, coming out to cause carnage in the chicken house, then going to earth again.

He was mortified to have lost Helicopter.

He was so keen to recapture the man that he hardly minded having to rely on the help of Willi Weber.

He just wanted to kill the fox.

It was a fine summer evening.

The car was parked at the northern end of the city.

Reims was a small town, and Dieter reckoned a car could drive from one side to the other in less than ten minutes.

He checked his watch: one minute past eight.

Helicopter was late coming on air.

Perhaps he would not broadcast tonight.

but that was unlikely.

Today Helicopter had met up with Michel.

As soon as possible, he would want to report his success to his superiors, and tell them just how much was left of the Bollinger circuit.

Michel had phoned the house in the rue du Bois two hours ago.

Dieter had been there.

It was a tense moment.

Stephanie had answered, in her imitation of Mademoiselle Lemas's voice.

Michel had given his code name, and asked whether "Bourgeoise" remembered him-a question that reassured Stephanie, because it indicated that Michel did not know Mademoiselle Lemas very well and therefore would not realize this was an impersonator.

He had asked her about her new recruit, codenamed Charenton.

"He's my cousin," Stephanie had said gruffly.

"I've known him since we were children, I would trust him with my life." Michel had told her she had no right to recruit people without at least discussing it with him, but he had appeared to believe her story, and Dieter had kissed Stephanie and told her she was a good enough actor to join the Comedie Francaise.

All the same, Helicopter would know that the Gestapo would be listening and trying to find him.

That was a risk he had to run: if he sent no messages home he was of no use.

He would stay on air only for the minimum length of time.

If he had a lot of information to send, he would break it into two or more messages and send them from different locations.

Dieter's only hope was that he would be tempted to stay on the air just a little too long.

The minutes ticked by.

There was silence in the car.

The men smoked nervously.

Then, at five past eight, the receiver beeped.

By prearrangement, the driver set off immediately, driving south.

The signal grew stronger, but slowly, making Dieter worry that they were not heading directly for the source.

Sure enough, as they passed the cathedral in the center of town, the needle fell back.

In the passenger seat, a Gestapo man talked into a short-wave radio.

He was consulting with someone in a radio-detection truck a mile away.

After a moment he said, "Northwest quarter." The driver immediately turned west, and the signal began to strengthen.

"Got you," Dieter breathed.

But five minutes had elapsed.

The car raced west, and the signal strengthened, as Helicopter continued to tap on the Morse key of his suitcase radio in his hiding place-a bathroom, an attic, a warehouse-somewhere in the northwest of the city.

Back at the chateau of Sainte-Cecile, a German radio operator had tuned to the same frequency and was taking down the coded message.

It was also being registered on a wire recorder.

Later, Dieter would decrypt it, using the one-time pad copied by Stephanie.

But the message was not as important as the messenger.

They entered a neighborhood of large old houses, mostly decrepit and subdivided into small apartments and bed sitting rooms for students and nurses.

The signal grew louder, then suddenly began to fade.

"Overshoot, overshoot!" said the Gestapo man in the front passenger seat.

The driver reversed the car, then braked.

Ten minutes had passed.

Dieter and the three Gestapo men sprang out.

The one with the portable detection unit under his raincoat walked rapidly along the pavement, consulting his wrist dial constantly, and the others followed.

He went a hundred meters, then suddenly turned back.

He stopped and pointed to a house.

"That one," he said.

"But the transmission has ended." Dieter noticed that there were no curtains in the windows.

The Resistance liked to use derelict houses for their transmissions.

The Gestapo man carrying the sledgehammer broke the door down with two blows.

They all rushed in.

The floors were bare and the place had a musty smell.

Dieter threw open a door and looked into an empty room.

Dieter opened the door of the back room.

He crossed the vacant room in three strides and looked into an abandoned kitchen.

He ran up the stairs.

On the next floor was a window overlooking a long back garden.

Dieter glanced out- and saw Helicopter and Michel running across the grass.

Michel was limping, Helicopter was carrying his little suitcase.

Dieter swore.

They must have escaped through a back door as the Gestapo were breaking down the front.

Dieter turned and yelled, "Back garden!" The Gestapo men ran and he followed.

As he reached the garden, he saw Michel and Helicopter scrambling over the back fence into the grounds of another house.

He joined in the chase, but the fugitives had a long lead.

With the three Gestapo men, he climbed the fence and ran through the second garden.

They reached the next street just in time to see a black Renault Monaquatre disappearing around the corner.

"Hell," Dieter said.

For the second time in a day, Helicopter had slipped through his grasp.