When Mother was ready, they went out. Their home stood in a row of tall, gracious town houses in the Mitte district, the old centre of the city, built for high-ranking ministers and army officers such as Carla’s grandfather, who had worked at the nearby government buildings.

Carla and her mother rode a tram along Unter den Linden, then took the S-train from Friedrich Strasse to the Zoo Station. The Francks lived in the south-western suburb of Schöneberg.

Carla was hoping to see Frieda’s brother Werner, who was fourteen. She liked him. Sometimes Carla and Frieda imagined that they had each married the other’s brother, and were next-door neighbours, and their children were best friends. It was just a game to Frieda, but secretly Carla was serious. Werner was handsome and grown-up and not a bit silly like Erik. In the doll’s house in Carla’s bedroom, the mother and father sleeping side by side in the miniature double bed were called Carla and Werner, but no one knew that, not even Frieda.

Frieda had another brother, Axel, who was seven; but he had been born with spina bifida, and had to have constant medical care. He lived in a special hospital on the outskirts of Berlin.

Mother was preoccupied on the journey. ‘I hope this is going to be all right,’ she muttered, half to herself, as they got off the train.

‘Of course it will,’ Carla said. ‘I’ll have a lovely time with Frieda.’

‘I didn’t mean that. I’m talking about my paragraph about Hitler.’

‘Are we in danger? Was Father right?’

‘Your father is usually right.’

‘What will happen to us if we’ve annoyed the Nazis?’

Mother stared at her strangely for a long moment, then said: ‘Dear God, what kind of a world did I bring you into?’ Then she went quiet.

After a ten-minute walk they arrived at a grand villa in a big garden. The Francks were rich: Frieda’s father, Ludwig, owned a factory making radio sets. Two cars stood in the drive. The large shiny black one belonged to Herr Franck. The engine rumbled, and a cloud of blue vapour rose from the tail pipe. The chauffeur, Ritter, with uniform trousers tucked into high boots, stood cap in hand ready to open the door. He bowed and said: ‘Good morning, Frau von Ulrich.’

The second car was a little green two-seater. A short man with a grey beard came out of the house carrying a leather case, and touched his hat to Mother as he got into the small car. ‘I wonder what Dr Rothmann is doing here so early in the morning,’ Mother said anxiously.

They soon found out. Frieda’s mother, Monika, came to the door; she was a tall woman with a mass of red hair. Anxiety showed on her pale face. Instead of welcoming them in, she stood squarely in the doorway as if to bar their entrance. ‘Frieda has measles!’ she said.

‘I’m so sorry!’ said Mother. ‘How is she?’