Aircraft work too, though, if one has the skill. Something about the concentration of people, I suppose. I sip my gin and tonic and look down at the clouds. The peaks of some of the higher mountains in the Norwegian coastal range protrude like jagged ice cubes floating in milk. I am taking a direct Great Circle route from London to Tokyo, cosseted within a giant aircraft coasting high above the weather where the sky is a deep, dark blue.

I may flit from here, within the plane. I may not. It is not an easy thing to do – many of us have wasted our drug by trying to effect transitions from remote places or – especially – moving start points. The way it appears to work is that if a successful flit cannot be made then nothing at all happens and one remains where one is. There are rumours, however, that people who have tried such manoeuvres have indeed ended up in another reality, but without the benefit of whatever mode of transport they left behind in the source reality being there to greet them in the target one. One pops into existence over open water if one flitted from a liner, splashing into an empty ocean to drown or be eaten by sharks, or – if the attempted transition is from an aircraft like this – one materialises in mid-air twelve thousand metres up with no air to breathe, a temperature of sixty below and a long way to fall. I have had successes flitting from aircraft, and failures; obviously failures where nothing happened.

I take the little ormolu case from my shirt pocket and turn it over and over on my fold-down table. To flit or not to flit. If I do vacate the aircraft then I will cover my tracks more completely than if I wait until my arrival. However, I could waste a pill. And I just might discover the hard way that the rumours are true, and find myself blast-frozen and gasping my way to unconsciousness as I start the long fall to the sea or the land. There is also the well-documented complication that sometimes one ends up in an aircraft going somewhere quite different to the destination of that one started from.

Usually there is a reliable commonality between a roughly aligned group of worlds regarding the placement of continents, major geographical features such as mountain ranges and rivers and hence big cities and therefore the air routes between them, so that leaving one aircraft results in a transition to a similar craft on a parallel course, but not always. There appear to be limits to the maximum displacement in space and time that people have made in such circumstances – a few kilometres up or down, a few dozen laterally, and some hours later or earlier – and it is as if some aspect of one’s will or visualisation is guiding one to the nearest approximation it’s possible to find, but sometimes the influence of this ghostly presence goes quite awry, or just accepts something that it hopes will do, but which will not.

Once, flitting while flying over the Alps bound for Napoli from Dublin, I ended up on a flight from Madrid to Kiev. That’s practically a right-angle! It took me a day and a half to repair the damage to my itinerary, and I missed one appointment. I had and have no idea why this happened. When I mentioned this little adventure to someone from the Transitionary Office – the primary body of l’Expédience, which at least in theory oversees all the actions of those like myself and Madame d’Ortolan – the bureaucrat concerned just blinked behind his rimless glasses and said how interesting this was and hastened to record a note! I mean, really.

The drug we take to effect our travels is called septus. Some take theirs in liquid form, from tiny vials like medical ampoules. Others prefer to snort their travellers’ medicine, or inject it. Some like it to be in the form of a suppository or pessary. Madame d’Ortolan was always said to have favoured the latter option.

I tap the little ormolu case gently on one corner, rotate it a quarter turn, tap it again, and repeat. Most of us take septus in pill form; it is simply less of a bother. I regard most of the other methods as being rather like showing off.

A clear patch of sea gleams up at me. A ship, made tiny by the vertical kilometres between us, slides slowly north across the ruffled grey surface, drawing a feathery white wake after it. I imagine somebody on that ship looking up and seeing this aircraft, a bright white dot leaving its own thin trail inscribed across the blue.

Perhaps some of those who are said to have disappeared are gone to other Earths entirely, where Pangaea still holds, Man never evolved and sapient otters or insectile hive-minds rule in our place – who can say?

When we flit we go to where we imagine, and if – distracted, disoriented – we imagine something too far away from what we know and where we wish to go to, we may end up somewhere it is somehow impossible to imagine one’s way back from. I don’t know how that could be – what saves people like myself, sometimes, is how intensely we long for our home – but you never know.

I have quizzed the theorists, technicians and general functionaries of the Transitionary Office regarding just how all of this works and have yet to receive a satisfactory answer. I am not supposed to know because I have no need to know. Still, I would like to know. My being sent to save that young doctor from being crushed in that collapsing building in Savoie, for example: does that not imply foresight? Must we – I mean the Concern – not have some ability to look ahead in time, or be able to use realities otherwise similar to another but separated only by being slightly displaced in time so that – having observed what has happened in the leading one – one is able to affect events in the trailing one? This would amount to the same thing.

Of course, maybe it was complete happenstance that the tenement collapsed, pure chance. I find this unlikely, however. Chance is rarely pure.

It was at the casino that I encountered Mrs Mulverhill again, for the first time in a long time – or at least so I thought. Not that I realised immediately.

Cities are, as I’ve said, the best places to flit between realities; nexuses of transportation in our multiple existence just as they are in any given single world. The principal embassy of l’Expédience in the world I have tended to travel to and within – partly though chance and partly through some affinitive predisposition on my part, I dare say – is in what is called variously Byzantium, Constantinople, Konstantiniyye, Stamboul or Istanbul, depending. It is an ideal focus for our interests and abilities, straddling continents, linking east and west and evoking the past and its manifold legacies in a way that few other cities do on this meta-Earth I deal with. Ancient, modern, a furious mix of peoples, faiths, histories and attitudes, poised above and threatened by myriad fault lines, it exemplifies both heritage, jeopardy, division and linkage all at once. We have another office in Jerusalem.

There used to be another, in Berlin, but that city has, perversely, become less attractive for our purposes since the fall of the Wall and the reunification of Germany (one of those distributed, straggling meta-events that resonated through the sheaved realities for all the many worlds like some coordinated spawning phenomenon). So the office was closed. A shame, in a way; I liked the old, divided Berlin, with its wall. The greater city was a vast, open, airy place enfolded with lakes and sprawling tracts of forest on both sides of the divide but still, at its core, there was always a forlorn air about it, as well as a faint feeling of imprisonment, on both sides.

And a slowly spinning plate, if you know what I mean. We look for spinning, wobbling plates; places where it feels that matters could go either way, where another spin, another input of energy might restore stability, but where, equally, just a little more neglect – or even a nudge in the right/wrong place – could produce catastrophe. There are interesting lessons to be gleaned from the wreckage that results. Sometimes you cannot tell everything about a thing until you’ve seen it broken.