His only hope now was a tremendous coup, a massive operation to round up the spies who were undermining the German war effort. So tonight he had set a trap for Werner Franck.

If Franck turned out to be innocent, he did not know what he would do.

In the front seat of the car, a walkie-talkie crackled. Macke’s pulse quickened. The driver picked up the handset. ‘Wagner here.’ He started the engine. ‘We’re on our way,’ he said. ‘Over and out.’

It had started.

Macke asked him: ‘Where are we headed?’

‘Kreuzberg.’ It was a densely populated low-rent neighbourhood south of the city centre.

As they pulled away, the air raid siren sounded.

That was an unwelcome complication. Macke looked out of the window. The searchlights came on, waving like giant wands. Macke supposed they must find planes sometimes, but he had never seen it happen. When the sirens ceased their howling, he could hear the thunder of approaching bombers. In the early years of the war, a British bombing mission had consisted of a few dozen aircraft – which was bad enough – but now they were sending hundreds at a time. The noise was terrifying even before they dropped their bombs.

Werner said: ‘I suppose we’d better call off our mission tonight.’

‘Hell, no,’ said Macke.

The roar of the planes grew.

Flares and small incendiary bombs began to fall as the car approached Kreuzberg. The neighbourhood was a typical target for the RAF’s current strategy of killing as many civilian factory workers as possible. With staggering hypocrisy Churchill and Attlee were claiming they attacked only military targets, and civilian casualties were a regrettable side effect. Berliners knew better.

Wagner drove as fast as he could along streets lit fitfully by flames. There were no people around apart from air raid officials: everyone else was legally obliged to take shelter. The only other vehicles were ambulances, fire engines and police cars.

Macke covertly studied Werner. The boy was edgy, never quite still, staring out of the window anxiously, tapping his foot in unconscious tension.

Macke had not confided his suspicions to anyone but his immediate team. It was going to be difficult for him if he had to admit that he had demonstrated Gestapo operations to someone who he now thought was a spy. He could end up under interrogation in his own basement torture chamber. He was not going to do it until he was sure. The only way he might get away with it would be if at the same time he could present his superiors with a captured spy.

But then, if his suspicion turned out to be true, he would arrest not just Werner but his family and friends, and announce the destruction of a massive spy ring. That would transform the picture. He might even be promoted.

As the raid progressed the type of bombs changed, and Macke heard the profound thudding sound of high explosive. Once the target was illuminated, the RAF liked to drop a mixture of large oil bombs to start fires and high explosive to ventilate the flames and hamper the emergency services. It was cruel, but Macke knew that the Luftwaffe’s bombing pattern was similar.