Payne walked to the centre of the hut and faced the sergeant, eyes serious, mouth pulled back, not quite in a grin.

‘On the other hand,’ McGregor continued, ‘my old buddy Charles is deployed behind a desk these days, so if I forget myself a little, ain’t no big shakes to the war effort.’

‘Still talking a good fight?’ Charles raised both hands, palms forward. ‘Words are cheap, my friend.’

‘Hoo-ah,’ someone said.

They all stepped back, grinning as they formed a ring.

McGregor and Payne circled, then tore into each other with open-hand blows – not so much slaps as punches with palm-heels – and thrusting knees, then Payne was behind McGregor, locking on a stranglehold. McGregor rolled over, taking Payne with him, twisting free as his shirt tore, but Payne came to his feet and jumped, heels coming down close to McGregor’s head—

‘Bronco, Sarge! Look out!’

—but he shifted and kicked from the ground, creating space to stand up, one hand outstretched as a guard. They closed again and then again, battering each other, hitting and grappling, until one of the soldiers called time. Then McGregor and Payne, sweat-slick and blood-covered, torsos heaving bellows-like, grinned at each other while the others whooped.

McGregor looked at Gavriela and winked.

You saw me watching.

She nodded back, then stepped away from the doorway, down to the dusty road, and headed for her temporary home, one hand on her stomach while her thoughts roiled, no longer understanding her place in the world or why she was here. From behind her sounded the diminishing yells of soldiers as they resumed their practice of hand-to-hand skills that with luck they would never have to use.

The party was pleasant, civilized and surreal. So many of the physicists were dispossessed Europeans that it might have been a celebration of peaceful escape, or even a conference back home before the bad days, with a sprinkling of American visitors in white shirts and broad ties, some of them loosened at the neck. Payne, in his dark suit, was once more the civilized being, with the beginnings of a facial bruise not yet turned obvious. He was talking to a tall man with flashing blue eyes who reminded Gavriela of Alan Turing, though his face and physique were longer, quite different.

A woman in a polka-dot sleeveless dress and white cotton gloves came to stand beside Gavriela. ‘Is that a sarsaparilla you’re drinking, dear?’

‘Yes, that’s all I wanted. I’m Gabby.’

‘And I’m Mary. Nice to meet you. That’s my husband over there.’

She indicated a thick-bodied man with heavy black glasses, unlit pipe in hand as he argued some point with colleagues.

‘I presume,’ added Mary, gesturing towards Payne, ‘that he’s your husband.’

‘Not at all,’ said Gavriela.

‘Oh. Well, look, let me introduce you to the other girls.’ Mary indicated a group of women sitting around a young, joker-faced man, the centre of their attention. ‘If we can distract them from Dick for a second or two, that is.’

In the usual random manner of party sounds, a region of quietness developed around Mary’s husband and the men he was arguing with, allowing words to carry.

‘—properties like angular momentum defining the limit of the liquid-drop model,’ he was saying. ‘So you might want to keep that in mind when you assume nuclear incompressibility.’

A blond man said, ‘You’re saying Bohr and Wheeler give wrong values for the fission barrier?’

‘I’m saying the absolute limit is too high compared to experimental—’

Gavriela walked away from Mary.

‘Excuse me,’ she said to the men. ‘I understood that Z-squared over A is the critical ratio only for symmetric spontaneous fission. It’s possible that asymmetric fission occurs for values under 50.’

‘Huh? Thank God,’ said the blond man, ‘that somebody gets the point. Given the probability of barrier penetration, the model predicts a gradual, not sudden introduction of—’

And then they were into it, in a manner which – to a reflective part of Gavriela’s mind – was not dissimilar to McGregor’s fight with Payne, despite the vastly different arena; but this time she was a participant, immersed in the argument as much as any of them. The wives’ frowns and wrinkled noses were a contrast to their husbands’ reaction: the men ignoring everything but the physics. It was after glancing at the women again that Gavriela noticed the man across the room, and then her concentration derailed.

No. Not here.

A thin man with brilliantine-slicked hair was talking as part of a group, not standing out from the others … except to one who could see the curls and twists of darkness surrounding him.

Then Payne was in front of her, blocking her view.

‘I’ve got someone you ought to meet,’ he told her. ‘If that’s all right.’

The others were already engaged in a new topic.

‘Of course.’

Dampness had sprung out over her face, and she dabbed at it as she left with Payne. Outside it was still darkening, less hot than before, but still too much for her. There was no conversation as Payne led her to one of the working huts, its interior lit; and they went inside to a plain office where a blue-eyed man was waiting.

‘Dr Oppenheimer,’ said Gavriela. ‘It’s good to meet you.’

‘Likewise, Dr Wolf.’ Oppenheimer smiled. ‘But I’ll call you Dr Woods or Gabby in public, I promise.’

‘They seem to think it’s best,’ she said.

‘Understood. Niels Bohr was Nicholas Baker while he was here.’ Oppenheimer seemed not to notice Payne’s frowning. ‘You know, he worked out that slow neutrons are the key to successful fission while he was sailing here from Europe.’

Gavriela had spent most of her voyage being sick, or perhaps that was an exaggerated memory.

‘So.’ Oppenheimer fiddled with his tie-clip, then stopped and focused his startling eyes on her. ‘We saw you flinch when you caught sight of Laszlo. It was a clear observation.’

‘Don’t worry,’ said Payne. ‘I don’t think he noticed you.’

The solid wooden furniture seemed to swirl.

‘What is this?’ she asked.

‘A rather direct confirmation’ – Oppenheimer sucked in a little air – ‘that this darkness phenomenon exists.’

‘I don’t—’

‘In the sense that I’ve never seen an atom directly, but everything from Brownian motion to X-ray crystallography tells me they’re unequivocally real. Not to mention the likelihood that we’ll be able to make a chain reaction soon.’

Payne was smiling a little.

‘Is this why Rupert sent me here?’ Gavriela said.

‘I don’t know any Rupert,’ said Payne, ‘and I sure as heck wouldn’t christen any son of mine with a name like that, but yeah, that’s why you’re here.’

Oppenheimer unfolded his narrow body and walked across the room, then back.

‘Actually, your other purpose is real enough.’ He glanced at Payne. ‘We’re happy to share our progress with you, though I’m rather sorry you have to go back to England. I think you’d be a fine addition to the project.’

It occurred to Gavriela that Turing, with his ferocious mind and his interest in fundamental physics, would be a boon to the work here; but she dared not suggest it, because he was needed where he was.

‘Ever the recruiter, eh?’ said Payne.

Oppenheimer grinned, and for the first time Gavriela realized what an egalitarian set-up he had managed to create at the heart of a wartime military establishment, even more so than Bletchley Park. But that was distracting her from the situation.

‘You knew something about this Laszlo already,’ she said.

‘We had a suspicion—’ Payne began.

‘There are others with your perceptual abilities, if not as acute.’ Oppenheimer’s eyes twinkled. ‘Perhaps even a mystic-minded introspective Dane had something to contribute on the matter.’

Payne was shaking his head.

‘Security, Robert. Security.’