The next morning, Nicolas writes a bread-and-butter letter to thank Rachkovsky for dinner and receives a chummy, handwritten reply by return of post. ‘A pleasure, my charming one. Come by for tea sometime.’ It’s signed with a single ambiguous initial, which can be read as a P for Peter in the Roman alphabet or an R for Rachkovsky in the Cyrillic one; it perches on a bed of curlicues.

Rachkovsky runs his kingdom from two modest basement offices in the Imperial Embassy at 79 Rue de Grenelle. This is the Paris headquarters of the so-called Third Section or Okhrana, the organisation set up in the aftermath of the Tsar’s assassination to maintain the security of the next monarch and his family. The sinister-sounding word simply means: protection.

Small but well funded and – under Rachkovsky – well organised, the Paris office controls agents across Europe, monitoring the activities of revolutionaries in exile. The primary threat is political terror. No one is in any doubt how dangerous that is.

Nicolas is given a laminated name badge by the receptionist and asked to wait while she tells her boss he’s there. Rachkovsky’s first act is to tut, remove the badge, and pitch it in the bin. ‘We won’t be needing that,’ he says. The receptionist, a modest, dark-haired young Russian woman, apologises profusely. ‘Don’t worry, Ksenia, Mr Notovitch is a personal friend. You weren’t to know.’

The basement is furnished with filing cabinets, a dedicated telegraph room, cipher books, equipment for perlustrating letters – steamers, bright lights, magnifying glasses – and trays of hectographic gel for copying them. Rachkovsky shows it all to him proudly, and talks about his plans to build a darkroom. ‘And that,’ he announces, ‘is the armoury. Do you like guns? I love them! Let’s have tea.’

They sit together in a glassed-in meeting space with a whiteboard. Along one wall are wanted bills for various agitators, and flow charts showing the interrelationships between the different factions of the revolutionary movements: anarchists, socialists, communists, syndicalists and so forth.

Nicolas is surprised that Rachkovsky is so open with his intelligence and says so. Rachkovsky waves his hand. ‘All old hat, dear boy. It’s like cutting heads off a hydra. No: that makes them sound too glamorous. They’re like a classroom of sixth-form girls: one minute besties, the next loathing each other. Sixth-form girls with a taste for savage violence.’

Rachkovsky’s frankness emboldens Nicolas to ask a question that has been bothering him: why me?

‘Why you? Dear boy!’ Rachkovsky still makes tea in the Russian way: he pours a syrupy decoction into the bottom of a glass and tops it up with hot water. ‘Why you? Reliable. Russian-speaking. Multilingual. With a great cover story – and an ex-Jew to boot. Professional endorsements from the great and the good and a track record from Macedonia to Madras. Why you? I can’t possibly imagine! Surely you mean to ask, What’s in it for me?’ Rachkovsky dithers over a box of chocolates, takes two, and pushes them towards Nicolas.