The North Sea wind was throwing everything it could muster at the coast road and Prince was drenched by the time he returned to the police station on Victoria Road. He’d gone for a walk to clear his head and the storm had certainly done that. Now he had an idea.

Before leaving Lincoln, the Chief Constable had taken Prince into his confidence, reaching up to lean unnecessarily close to him, his bad breath causing Prince to pull away.

‘Entre nous, I’m pretty sure that chap is MI5 – or MI6, one of the two.’ He’d coughed as he said ‘MI5’ and mispronounced entre as entray, and he’d clearly hoped Prince would be more impressed than he evidently was.

‘I know, sir. Of course he is.’

‘You knew?’

‘I knew he couldn’t be anything else.’

The Scotsman told Prince he could call him Douglas, though it was unclear whether this was an assumed first name or an assumed surname. They’d been in Mablethorpe for three days and were no nearer to finding the German agent, and they were close to admitting that the German must have left the area. He’d disappeared.

Their one last shot was a plan to draft in officers from across the county and visit every house in the area on the pretext of searching for a missing soldier. They’d come up with a story that the soldier was Norwegian, which they hoped would alert people to and account for a foreign accent. Though Prince thought if the agent had managed to stay in hiding for a week, a knock at the door and a few questions about a missing Norwegian soldier would be unlikely to flush him out.

But as he was sprayed with half of the North Sea and much of the sand from the beach, he had another idea. In 1938, he’d been asked to compile a list of political extremists in the county. The communists were easy enough because someone had already helpfully provided them with a list. The fascists were a bit harder, even once he’d managed to persuade some sceptical senior officers that they were indeed a threat. Then he’d had a brainwave: the British Union of Fascists had a newspaper called Action, which was posted to members every week. Prince had alerted the postal sorting offices and within a fortnight had a detailed list of all recipients in the county.

Once back in the police station he made a call, and an hour later Inspector Lord arrived from the divisional headquarters in Skegness.

‘You’ve brought the files?’

Lord placed a manila file bound with white string on the desk, the words Fascists 1938/9 typed on a peeling label on the front.

‘And this is up to date?’