At last, Hepton had two clear days free from the base, and could take a drive into the country. Ripening fields, the sun beating down as it had no right to do in the course of an English summer. An occasional splash of garish yellow where rape – vegetable cash to the farmers – had been sown. But mostly the fields were green, or were delivering up golden buds of wheat and barley. A beautiful country. He so seldom noticed it, but it was the truth. He had become blind through living his life underground, but like a mole, he had burrowed his way to the surface and was now scenting the air anew. He checked in the Renault’s rear-view mirror. The Ford Sierra was still with him, a hundred yards back but quite noticeable, there being so little traffic on these winding roads.

Twenty minutes ago, he had slowed behind a tractor, though overtaking would have been easy, and had watched the car behind him edge forward until the face of the driver – female – was clear in his mirror. The face of a businesswoman, but she didn’t appear to be in any rush. So that when he had waved her past, she had flashed her lights once in acknowledgement, but shaken her head too. And stayed behind him.

She was still behind him, sometimes gaining, sometimes losing. He thought of pulling in to a café, to see whether she would follow him. After all, this might be innocence itself, a chance encounter that might lead … well, anywhere. Except that he had to press on. Paul Vincent was in the Alfred de Lyon Hospital, and the Alfred de Lyon Hospital was an inconvenient forty or so miles away yet. So he drove on.

He thought of Zephyr, of the miracles satellites could perform. He stuck an arm out of his window and waved towards the sky, wondering if he could be seen. There was no doubt that Zephyr could see him if it wanted to. It could give close-ups of his car, of his number plate. But Zephyr wasn’t trained on him. It was trained on a series of air force bases, where the US personnel were preparing for their flights back home. Or at least that was what it had been watching yesterday evening, on Hepton’s last shift.

Something niggled, though, something he had noticed and mentioned in passing to Nick Christopher. It was to do with Buchan Air Force Base in the north of Scotland, just outside the town of Peterhead. Buchan had been an RAF radar station, but then the American forces had moved in for a time. Hepton had watched it before. He liked the quality of light in northern Scotland. That was what niggled: the sun was setting early in Buchan.

‘Cloud cover,’ Christopher had explained. ‘I’ve seen the weather reports. Overcast skies.’

‘Yes,’ Hepton had said, ‘but they’re not overcast.’