‘I tell my boss it’s for official business,’ he said. Werner worked for an important general. ‘But I don’t know how much longer I can get away with it.’

The von Kessel family lived in the same suburb. Werner drove there in five minutes.

The house was luxurious, though smaller than the Francks’. Heinrich met them at the door and showed them into a living room with leather-bound books and an old German woodcarving of an eagle.

Frieda kissed him. ‘Thank you for doing this,’ she said. ‘It probably wasn’t easy – I know you don’t get on so well with your father.’

Heinrich beamed with pleasure.

His mother brought them coffee and cake. She seemed a warm, simple person. When she had served them she left, like a maid.

Heinrich’s father, Gottfried, came in. He had the same thick straight hair, but it was silver instead of black.

Heinrich said: ‘Father, here are Werner and Frieda Franck, whose father manufactures People’s Radios.’

‘Ah, yes,’ said Gottfried. ‘I have seen your father in the Herrenklub.’

‘And this is Carla von Ulrich – I believe you know her father, too.’

‘We were colleagues at the German embassy in London,’ Gottfried said carefully. ‘That was in 1914.’ Clearly he was not so pleased to be reminded of his association with a social democrat. He took a piece of cake, clumsily dropped it on the rug, tried ineffectually to pick up the crumbs, then abandoned the effort and sat back.

Carla thought: What is he afraid of?

Heinrich got straight down to the purpose of the visit. ‘Father, I expect you’ve heard of Akelberg.’

Carla was watching Gottfried closely. There was a split-second flash of something in his expression, but he quickly adopted a pose of indifference. ‘A small town in Bavaria?’ he said.

‘There is a hospital there,’ said Heinrich. ‘For mentally handicapped people.’

‘I don’t think I was aware of that.’

‘We think something strange is going on there, and we wondered if you might know about it.’

‘I certainly don’t. What seems to be happening?’

Werner broke in. ‘My brother died there, apparently of appendicitis. Herr von Ulrich’s maid’s child died at the same time in the same hospital of the same illness.’

‘Very sad – but a coincidence, surely?’

Carla said: ‘My maid’s child did not have an appendix. It was removed two years ago.’

‘I understand why you are keen to ascertain the facts,’ said Gottfried. ‘This is deeply unsatisfactory. However, the likeliest explanation would seem to be clerical error.’

Werner said: ‘If so, we would like to know.’

‘Of course. Have you written to the hospital?’

Carla said: ‘I wrote to ask when my maid could visit her son. They never replied.’

Werner said: ‘My father telephoned the hospital this morning. The Senior Physician slammed the phone down on him.’

‘Oh, dear. Such bad manners. But, you know, this is hardly a Foreign Office matter.’

Werner leaned forward. ‘Herr von Kessel, is it possible that both boys were involved in a secret experiment that went wrong?’