“Don’t you get carried away there now, know what I mean?”

She shrugged. “There are couriers who can only take another person with them when they are penetratively conjoined. I have to embrace my fellow traveller. One or two can co-transition just by holding the other’s hand. Anyway. We’ll see. All I’m saying is, don’t be alarmed if we flit back and that’s what we’re doing.”

“Okay,” I said. “I’ll try not to be alarmed.”

She stepped up to me. “Now we embrace, yes?”

My brain felt like it was turning inside out again. Or outside in this time. Whatever. But when we got back I was lying curled up on the floor of the amber-lit room and Mrs M was sitting cross-legged by my side, patting my shoulder and making sorrowful, comforting noises and I had tears in my eyes and a sick feeling in my gut, nursing what felt like a pair of badly bruised testicles, exactly as though somebody had kneed me in the balls a few minutes earlier.

“Ah,” she said. “Sorry. Sometimes that happens, too.”

Infinities within infinities within infinities… The human brain quails when confronted with such proliferating vastness. We think we have a grasp of it, brandishing our numbers – natural, rational, complex, real, unreal – in the face of all that’s inestimable, but truthfully these resources are mere talismans, not practical tools. A comfort; no more.

Nevertheless, the doorways into that inexhaustible wilderness of forever multiplying worlds had been opened to us, and we required the means to at least try to understand as much as we could of their hidden mechanisms and how they might be comprehended and navigated.

Learning about the many worlds occurred, appropriately, in layers. One was history. In at least three categories.

There was history that we knew we were allowed to know, history that we knew we were not allowed to know, and history that allegedly didn’t exist but that we – that is, the students of this effectively measureless subject – suspected did exist but was never talked about, not at our level and perhaps not even at the level of the people who taught us.

We were aware from the beginning that the Concern had many more levels than were immediately visible from the lowly strata where we existed in its tortuously convoluted hierarchy, and it was hard even to guess at how far beyond us it extended, given both the irredeemably complex nature of the many worlds themselves and the seemingly quite deliberate opacity of the organisation’s structure.

We knew there were various levels and classes of executives within l’Expédience with, at the apparent pinnacle of this structure, the Central Council itself, composed of people who knew all there was to know about the Concern’s provenance, internal configuration, extent, operational methods and aims, and some of us were of the opinion – always perverse, in mine – that there might be one central authority figure at the head of all this tiered knowledge and power, a kind of organisational autocrat to whom everybody else was obliged to defer. But for all we knew that final, single, near-godlike Emperor of the realities – if he or she did exist – was little better than a foot soldier in a still greater grouping of other Concerns and meta-Concerns extending further and higher out across and through the furiously expanding realities and numbered in millions, billions, trillions… who knew?

For us lowly foot soldiers, though, mere trainees that we were, the centre of our world – the centre of all our many worlds – was the Speditionary Faculty of the University of Practical Talents, Aspherje, on an Earth that – almost uniquely – did not call itself Earth, but Calbefraques.

Calbefraques was the ultimate Open world, the mirror image of one of the numberless perfectly Closed Earths where nobody knew about the many worlds; a place where possibly every single adult soul who walked its surface knew that it was merely one world within an infinitude of worlds, and a nexus at that, a stepping-off point for as much of that infinitude as it was possible to imagine.

And a world, an Earth that was close to unique. Logically there had to be other versions of this Earth that were close to the Calbefraques that we knew, but we seemed to be unable to access them. It was as though by being the place that could act as a gateway to any other version of Earth, Calbefraques had somehow outpaced all the other versions of itself that would otherwise have existed. It seemed that in the same way that the true consciousness of a transitioner could only be in one world at a time, there could only be one world that was perfectly Open, and that world, that unique Earth was this one, called Calbefraques.

It was here that almost all the transitioners lived when they were not on missions to other worlds, and here too that the vast majority of theorists of transitioning, experts in transitioning, researchers into transitioning and experimental practitioners of transitioning both made their home and plied their trade. In its globally distributed factories and laboratories all the multifarious paraphernalia of transitioning was manufactured, and – somewhere, allegedly – the ultimately precious substance we called septus, the drug that made flitting possible in the first place, was brought into being. Exactly how and where this was done and exactly what septus really was, nobody seemed to know. The secrecy surrounding the drug’s creation was of an order more intense even than that associated with the severely security-conscious operations of the Transitionary Corps. Naturally, this meant that the speculation regarding this piece of arcana was, to put it mildly, unrestrained.

There were strict rules about the use and exposure of septus within this world or any other, restricting its use to its flitting-enabling purpose and absolutely nothing else. But it was rumoured that, if one did try to have some of it analysed, in the most advanced laboratories one could find, the sample itself simply vanished, or appeared on inspection – by chemical analysis, mass spectrometry, microscopes working on a variety of wavelengths or any other technique available – to be nothing more complicated than pond slime, or even pure water.

Here, in the university that was a city within a city, within its piled pyramids, ziggurats, towers and colonnades, and in the profusion of outlying buildings distributed all across the greater city – an ever-multiplying number, in a fit image of what was studied within them – millions of students like myself had, over the years, learned as much of that proportion of the truth as it was thought appropriate for us to be allowed to comprehend. What some of us really wanted to know, naturally, was the size of that proportion, and what was concealed in the fraction of it being denied us.

It was the septennial Festival of Death in Aspherje, Calbefraques, and the Central Council of the Transitionary Office had arranged a particularly extravagant party and ball to celebrate both the formal cultural event and the latest expansion and reconstitution of the Council.

Guests arrived on a specially constructed narrow-gauge railway which ran in a loop round the closed city centre, picking up guests from a variety of temporary stations – manned by servants dressed as ghouls – which were dotted around the periphery of the cordoned-off area, where the guests’ own transport had deposited them. The track was lit by tall, smokily guttering torches and by burning braziers hanging from gibbets and made to look like ancient roadside punishment cages, the skeletons of starved miscreants visible through the smoke and flames inside.