Several people hastened to get him a glass of vodka. Grigori insisted on giving Stalin his chair in the centre of the head table. The buzz of conversation resumed, but it was subdued: they were thrilled he was here, but now they had to be careful of every word and every move. This man could have a person killed with a snap of his fingers, and he frequently had.

More vodka was brought, the band began to play Russian folk dances, and slowly people relaxed. Volodya, Zoya, Grigori and Katerina did a four-person dance called a kadril, which was intended to be comic and always made people laugh. After that more couples danced, and the men started to do the barynya, in which they had to squat and kick up their legs, which caused many of them to fall over. Volodya kept checking on Stalin out of the corner of his eye – as did everyone else in the room – and he seemed to be enjoying himself, tapping his glass on the table in time with the balalaikas.

Zoya and Katerina were dancing a troika with Zoya’s boss, Vasili, a senior physicist working on the bomb project, and Volodya was sitting out, when the atmosphere changed.

An aide in a civilian suit came in, hurried around the edge of the room, and went right up to Stalin. Without ceremony, he leaned over the leader’s shoulder and spoke to him quietly but urgently.

Stalin at first looked puzzled, and asked a sharp question, then another. Then his face changed. He went pale, and seemed to stare at the dancers without seeing them.

Volodya said under his breath: ‘What the hell has happened?’

The dancers had not yet noticed, but those sitting at the head table looked frightened.

After a moment Stalin stood up. Those around him deferentially did the same. Volodya saw that his father was still dancing. People had been shot for less.

But Stalin had no eyes for the wedding guests. With the aide at his side he left the table. He walked towards the door, crossing the dance floor. Terrified revellers jumped out of his way. One couple fell over. Stalin did not seem to notice. The band ground to a halt. Saying nothing, looking at nobody, Stalin left the room.

Some of the generals followed him out, looking scared.

Another aide appeared, then two more. They all sought out their bosses and spoke to them. A young man in a tweed jacket went up to Vasili. Zoya seemed to know the man, and listened intently to him. She looked shocked.

Vasili and the aide left the room. Volodya went to Zoya and said: ‘For God’s sake, what’s going on?’

Her voice was shaky. ‘The Americans have dropped a nuclear bomb in Japan.’ Her beautifully pale face seemed even whiter than normal. ‘At first the Japanese government couldn’t figure out what had happened. It took them hours to realize what it was.’

‘Are we sure?’

‘It flattened five square miles of buildings. They estimate that seventy-five thousand people were killed instantly.’