Prince glanced around him. He’d been shocked enough on the streets of Denmark at the ubiquity of the Nazi occupation: the flags, the soldiers, the weapons. But this was altogether different and considerably more sinister than Denmark. At least there it was reasonable to assume that most people around him at a checkpoint or in the street were opposed to the Nazis. It was hard to underestimate how reassuring a slightly raised eyebrow, a frown or a hint of a smile could be. But here he had to assume everyone he encountered was the enemy: the mother pushing a pram, the old lady weighed down by her shopping, the limbless veteran of the Great War. It was far more unsettling than he’d anticipated.

They were now on Anhalter Strasse, with Anhalter station – the gateway to Munich and Frankfurt – to their left and on their right an enormous and rather ornate building.

‘That building on your right – please don’t stare at it, just keep looking ahead or at me – is the headquarters of the Gestapo. This is the rear of the building; the main entrance is on Prinz Albrecht Strasse. The high iron gates we’re just walking past are where prisoners are driven in. They say very few who enter that way leave it alive. Here’s our hotel.’

Prince was overwhelmed by the size of Hotel Excelsior as they climbed the steps to the entrance. Armed soldiers in grey greatcoats stood on either side of enormous doors. From the parapets hung flags the size of houses, giant swastikas set against a blood-red background, draped like the sails on a Viking ship.

‘Five hundred rooms, nine restaurants, the largest hotel in Berlin,’ said Otto. ‘I like to think there’s a degree of anonymity in staying here, but I don’t doubt I’m deluding myself. There are Gestapo everywhere.’

While they waited to check in, Prince noticed a woman standing behind them, apparently studying a noticeboard with that day’s menus pinned to it. He’d glimpsed her as they walked into reception and something about her caught his attention, but he couldn’t be sure what it was. He turned round again, and this time the woman was facing the reception desk, though not looking in his direction. She had extraordinary dark eyes that darted around, and was wearing a turban-style hat. Ignoring Prince and Knudsen, she caught the attention of the reception clerk they were about to deal with. She spoke in a gentle accent, quite a cultured one. Could he check once more about her table reservation, please?

They signed the register and were allocated rooms next to each other on the fourth floor, overlooking Saarland Strasse. Before they went upstairs, the concierge called them over.

‘Have you been in Berlin during an air raid?’

Both men shook their heads. Prince resisted the temptation to mention that he’d most probably seen the bombers as they left their Lincolnshire bases.