Jacky brought the desserts and asked if they wanted coffee. They did not have time – the army did not believe in long lunch breaks – and Margaret asked for the bill. ‘Guests aren’t allowed to pay here,’ she explained.

When Jacky had gone, Margaret said: ‘What’s nice is that you’re so fond of her.’

‘Am I?’ Greg was surprised. ‘I have fond memories, I guess. I wouldn’t mind being fifteen again.’

‘And yet she’s scared of you.’

‘She is not!’

‘Terrified.’

‘I don’t think so.’

‘Take my word. Men are blind, but a woman sees these things.’

Greg looked hard at Jacky when she brought the bill, and he realized that Margaret was right. Jacky was still scared. Every time she saw Greg she was reminded of Joe Brekhunov and his straight razor.

It made Greg angry. The girl had a right to live in peace.

He was going to have to do something about this.

Margaret, who was as sharp as a tack, said: ‘I think you know why she’s scared.’

‘My father frightened her off. He was worried I might marry her.’

‘Is your father scary?’

‘He does like to get his own way.’

‘My father’s the same,’ she said. ‘Sweet as cherry pie, until you cross him. Then he turns mean.’

‘I’m so glad you understand.’

They returned to work. Greg felt angry all afternoon. Somehow his father’s curse still lay like a blight over Jacky’s life. But what could he do?

What would his father do? That was a good way to look at it. Lev would be completely single-minded about getting his way, and would not care who he hurt in the process. General Groves would be similar. I can be like that, Greg thought; I’m my father’s son.

The beginning of a plan began to form in his mind.

He spent the afternoon reading and summarizing an interim report from the University of Chicago Metallurgical Laboratory. The scientists there included Leo Szilard, the man who first conceived of the nuclear chain reaction. Szilard was a Hungarian Jew who had studied at the University of Berlin – until the fatal year of 1933. The research team in Chicago was led by Enrico Fermi, the Italian physicist. Fermi, whose wife was Jewish, had left Italy when Mussolini published his Manifesto of Race.

Greg wondered whether the Fascists realized that their racism had brought such a windfall of brilliant scientists to their enemies.

He understood the physics perfectly well. The theory of Fermi and Szilard was that when a neutron struck a uranium atom, the collision could produce two neutrons. Those two neutrons could then collide with further uranium atoms to make four, then eight, and so on. Szilard had called this a chain reaction – a brilliant insight.

That way, a ton of uranium could produce as much energy as three million tons of coal – in theory.

In practice, it had never been done.

Fermi and his team were building a pile of uranium at Stagg Field, a disused football stadium belonging to the University of Chicago. To prevent the stuff exploding spontaneously, they buried the uranium in graphite, which absorbed the neutrons and killed the chain reaction. Their aim was to bring the radioactivity up, very gradually, to the level at which more was being created than absorbed – which would prove that a chain reaction was a reality – then close it down, fast, before it blew up the pile, the stadium, the campus of the university, and quite possibly the city of Chicago.