When Volodya reached the headquarters building at the Khodynka airfield, he found the air full of greyish flakes that were not snow but ash. Red Army Intelligence was burning its records to prevent them falling into enemy hands.

Shortly after he arrived, Colonel Lemitov came into his office. ‘You sent a memo to London about a German physicist called Wilhelm Frunze. That was a very smart move. It turned out to be a great lead. Well done.’

What does it matter, Volodya thought? The Panzers were only a hundred miles away. It was too late for spies to help. But he forced himself to concentrate. ‘Frunze, yes. I was at school with him in Berlin.’

‘London contacted him and he is willing to talk. They met at a safe house.’ As Lemitov talked, he fiddled with his wristwatch. It was unusual for him to fidget. He was clearly tense. Everyone was tense.

Volodya said nothing. Obviously some information had come out of the meeting, otherwise Lemitov would not be talking about it.

‘London say that Frunze was wary at first, and suspected our man of belonging to the British secret police,’ Lemitov said with a smile. ‘In fact, after the initial meeting he went to Kensington Palace Gardens and knocked on the door of our embassy and demanded confirmation that our man was genuine!’

Volodya smiled. ‘A real amateur.’

‘Exactly,’ said Lemitov. ‘A disinformation decoy wouldn’t do anything so stupid.’

The Soviet Union was not finished yet, not quite; so Volodya had to carry on as if Willi Frunze mattered. ‘What did he give us, sir?’

‘He says he and his fellow scientists are collaborating with the Americans to make a super-bomb.’

Volodya, startled, recalled what Zoya Vorotsyntsev had told him. This confirmed her worst fears.

Lemitov went on: ‘There’s a problem with the information.’

‘What?’

‘We’ve translated it, but we still can’t understand a word.’ Lemitov handed Volodya a sheaf of typewritten sheets.

Volodya read a heading aloud. ‘Isotope separation by gaseous diffusion.’

‘You see what I mean.’

‘I did languages at university, not physics.’

‘But you once mentioned a physicist you know.’ Lemitov smiled. ‘A gorgeous blonde who declined to go to a movie with you, if I remember.’

Volodya blushed. He had told Kamen about Zoya, and Kamen must have repeated the gossip. The trouble with having a spy for a boss was that he knew everything. ‘She’s a family friend. She told me about an explosive process called fission. Do you want me to question her?’

‘Unofficially and informally. I don’t want to make a big thing of this until I understand it. Frunze may be a crackpot, and he could make us look foolish. Find out what the reports are about, and whether Frunze is making scientific sense. If he’s genuine, can the British and Americans really make a super-bomb? And the Germans too?’

‘I haven’t seen Zoya for two or three months.’

Lemitov shrugged. It did not really matter how well Volodya knew Zoya. In the Soviet Union, answering questions put by the authorities was never optional.