McHugh was probably a Communist. Greg knew this because J. Robert Oppenheimer had told him. Oppenheimer, a leading theoretical physicist from the University of California, was director of the Los Alamos laboratory and scientific leader of the entire Manhattan Project. He had strong Communist ties, though he insisted he had never joined the party.

Special Agent Bicks had said to Greg: ‘Why does the army have to have all these pinkos? Whatever it is you’re trying to achieve out there in the desert, aren’t there enough bright young conservative scientists in America to do it?’

‘No, there aren’t,’ Greg had told him. ‘If there were, we would have hired them.’

Communists were sometimes more loyal to their cause than to their country, and might think it right to share the secrets of nuclear research with the Soviet Union. This would not be like giving information to the enemy. The Soviets were America’s allies against the Nazis – in fact, they had done more of the fighting than all the other allies put together. All the same it was dangerous. Information intended for Moscow might find its way to Berlin. And anyone who thought about the postwar world for more than a minute could guess that the USA and the USSR might not always be friends.

The FBI thought Oppenheimer was a security risk and kept trying to persuade Greg’s boss, General Groves, to fire him. But Oppenheimer was the outstanding scientist of his generation, so the General insisted on keeping him.

In an attempt to prove his loyalty, Oppenheimer had named McHugh as a possible Communist, and that was why Greg was tailing him.

The FBI were sceptical. ‘Oppenheimer is blowing smoke up your ass,’ Bicks had said.

Greg said: ‘I can’t believe it. I’ve known him for a year now.’

‘He’s a fucking Communist, like his wife and his brother and his sister-in-law.’

‘He’s working nineteen hours a day to build better weapons for American soldiers – what kind of traitor does that?’

Greg hoped McHugh did turn out to be a spy, for that would lift suspicion from Oppenheimer, bolster General Groves’s credibility, and boost Greg’s own status too.

He watched McHugh constantly throughout the first half of the concert, not wanting to take his eyes off him. The physicist did not look at the people either side of him. He seemed absorbed in the music, and only moved his gaze from the stage to look lovingly at Mrs McHugh, who was a pale English rose. Had Oppenheimer simply been wrong about McHugh? Or, more subtly, was Oppenheimer’s accusation a distraction to divert suspicion away from himself?

Bicks was watching, too, Greg knew. He was upstairs in the dress circle. Perhaps he had seen something.

In the interval, Greg followed the McHughs out and stood in the same line for coffee. Neither the dowdy couple nor the two old ladies were anywhere nearby.

Greg felt thwarted. He did not know what to conclude. Were his suspicions unfounded? Or was it simply that this visit by the McHughs was innocent?