A cancer specialist in a white coat approached. As he went by, Carla said brightly to Beck: ‘How are things?’

‘I’m as fit as I’m ever going to be. I’ll never lead a battalion into battle again, but aside from athletics I can lead a normal life.’

‘I’m glad to hear that.’

People kept walking by. Carla feared Beck would never get the chance to say anything to her privately.

But he remained unruffled. ‘I’d just like to thank you for your kindness and professionalism.’

‘You’re welcome.’

‘Goodbye, Sister.’

‘Goodbye, Colonel.’

When Beck left, Carla was holding the file folder.

She walked briskly to the nurses’ cloakroom. It was empty. She stood with her heel firmly wedged against the door so no one could come in.

Inside the folder was a large envelope made of the cheap buff-coloured paper used in offices everywhere. Carla opened the envelope. It contained several typewritten sheets. She looked at the first without removing it from the envelope. It was headed:

OPERATIONAL ORDER NO. 6

CODE ZITADELLE

It was the battle plan for the summer offensive on the Eastern Front. Her heart raced. This was gold dust.

She had to pass the envelope to Frieda. Unfortunately, Frieda was not working at the hospital today: it was her day off. Carla considered leaving the hospital right away, in the middle of her shift, and going to Frieda’s house; but she swiftly rejected that idea. Better to behave normally, not to attract attention.

She slipped the envelope into the shoulder bag hanging on her coat hook. She covered it with the blue-and-gold silk scarf that she always carried for hiding things. She stood still for a few moments, letting her breathing return to normal. Then she went back to the ward.

She worked the rest of her shift as best she could, then she put on her coat, left the hospital, and walked to the station. Passing a bomb site, she saw graffiti on the remains of the building. A defiant patriot had written: ‘Our walls might break, but not our hearts.’ But someone else had ironically quoted Hitler’s 1933 election slogan: ‘Give me four years, and you will not recognize Germany.’

She bought a ticket to the Zoo.

On the train she felt like an alien. All the other passengers were loyal Germans, and she was the one with secrets in her bag to betray to Moscow. She did not like the feeling. No one looked at her, but that only made her think they were all deliberately avoiding her eye. She could hardly wait to hand over the envelope to Frieda.

The Zoo Station was on the edge of the Tiergarten. The trees were dwarfed, now, by a huge flak tower. One of three in Berlin, this square concrete block was more than 100 feet high. At the corners of the roof were four giant 128mm anti-aircraft guns weighing 25 tons each. The raw concrete was painted green in a hopelessly optimistic attempt to make the monstrosity less of an eyesore in the park.