At the frontier he was shaken to wakefulness by a diminutive Prussian customs inspector, accompanied by a uniformed and helmeted policeman. Swallow reckoned they both smelled of drink.

‘Papers bitte.’

He opened the Home Office travel letter with which Mallon’s clerk had provided him. He had requested that it should not detail his occupation or the reason for his travel. He held it up to the inspector.

‘Ah, Englander. Auf geschäft? On business?’

Swallow nodded.

‘Yes. On business.’

‘Your destination?’

‘Berlin.’

The official cocked his head to one side.

‘The nature of your business? Your exact destination?’

The tone was hostile. Swallow could smell the alcohol distinctly now.

‘I’d rather not say. Private business.’

He could see the inspector’s face redden angrily in the dawn light.

‘You will answer,’ the man barked, jabbing a finger towards his uniformed companion, ‘or it becomes a matter for the police.’

‘I have shown you my travel letter,’ Swallow answered quietly. ‘It is quite in order.’

‘I will not ask again,’ the inspector hissed. The policeman’s right hand moved towards his holstered pistol.

Suddenly, the man who had been sitting silently opposite since the train had departed the Hook rose from his seat to place himself between Swallow and the pair.

He thrust a black, leather-covered wallet in front of the inspector’s face.

‘Ja, das ist eine Sache für die Polizei. Ich bin ein Offizier der Kriminalpolizei.’

Swallow had not understood the language, but it was clear enough what was going on. The inspector took a step back and bowed slightly.

‘Apologies, mein herr. Ich wusste nicht. I did not realise. . . .’

Swallow’s rescuer winked at him under his brimmed hat as the two moved awkwardly to the door, bowing again in unison. He held his hand out, smiling.

‘Pfaus. Berlin Kriminalpolizei. I’m sorry you were bothered by those two stupid drunks.’

He resumed his seat across the carriage. Swallow could not conceal his surprise.

‘You’re Pfaus? Then you know who I am.’

‘Of course, Mr Joseph Swallow. You may call me Johann. I believe that is not inappropriate given the equivalence in our ranks. Or we can address each other by rank.’

His English was perfect. The accent, Swallow thought, had an American trace.

‘Well, Johann, or Kapitän Pfaus, this is a bit of a surprise.’

Pfaus grinned again. He removed his hat and stretched his legs as if he had decided to relax, his identity having been revealed.

‘My superiors decided we couldn’t have a senior British police officer coming to make a visit without taking care of him on his journey.’

Swallow smiled back.

‘So are you travelling with me to protect me or to keep me under observation?’

Pfaus grinned again. He drew a flat pewter flask from the pocket of his greatcoat, unscrewed the top and held it out.