Only an hour after landing at Kourou Stef was being bundled into a small aircraft for her hop across the Atlantic. Like the shuttle the plane was crowded, fully loaded before it was allowed to take off; these days transport was always communal and always crowded, planes and shuttles and trains and buses, minimal energy usage the key goal.

The plane, powered by turbos driven by a compact microfusion engine, leapt easily into the air. The sky beyond the small, thick windows turned a deep blue; the trajectory was a suborbital hop, and they crossed the Atlantic high and fast, heading north-east towards western Europe, Portugal and Spain.

As the plane dipped back into the atmosphere over the Iberian coast, Stef wished she knew enough geography to recognise how much of this coastline had been changed by the risen sea. Near the shore she saw vapour feathers gleaming white, artificial cloud created by spray turbines standing out to sea and deflecting a little more sunlight from the overheated Earth. The ocean itself was green-blue, thick with plankton stimulated to grow and draw down carbon from the air.

The plane banked and headed north, streaking at high speed through the air. Southern Spain, long abandoned to desert, was chrome-plated with solar-cell farms, and studded with vast silvered bubbles, lodes of frozen-out carbon dioxide. Once across the Pyrenees they left behind the mid-latitude desertification zone and the ground gradually became greener. But even in central France Stef glimpsed great old cities abandoned or at least depopulated, the conurbations’ brown stains pierced by green as they fragmented back into the villages from which they had formed. Over northern France the plane swept west, circled, and then descended into an easterly headwind. Stef saw something of the Seine, more abandoned towns on a glistening floodplain. Away from the river olives grew in neat rows on dusty ground, a sight you would once have seen in southern Spain, an agriculture suited to the new age.

At Paris, the big old airports were no longer in use. Instead, with a stab of sharp deceleration, they were brought down at a small airport in a suburb called Bagneux, just south of central Paris itself, a clutter of ugly twentieth-century buildings cleared in great stripes to make room for the runways. There was another brisk transfer made mostly in silence; there was no documentation, but every passenger’s identity, security background and infective status were seamlessly checked with non-invasive DNA scans.

Soon Stef was through the process, and found herself and her minimal luggage alone in a small driverless electric car that whisked her north towards the city centre. She’d not been to Paris before, and the cramped streets swept by at bewildering speed. Somewhere the Eiffel Tower still stood proud, but around her she saw only walls of ancient sandstone stained by floodwater. Although this was still the political capital of the country there were few people around. Wealthy Parisians had long ago decanted to the cooler climes of southern England – Angleterre as it was known now – and the poor, presumably, had died out or drifted away.

She glimpsed the Île de la Cité, standing in the turbid waters of the Seine, where the roofs of Notre Dame were plastered with solar panels. A huge banyan tree sprawled before the cathedral, rooted in the flooded ruins of surrounding buildings.

At last the car brought her to the Champs-Élysées, an avenue even a first-time visitor like Stef could not fail to recognise. There was a fair density of traffic here, and pedestrians hurrying beneath sun-shade awnings. The car stopped outside a high, elaborate doorway, where a man stood in the shade, beside a slim woman in the uniform of the ISF. The man, of course, was Earthshine. And the woman was Penny Kalinski, Stef’s impossible sister.

Earthshine, who cast a convincing shadow when he stepped into the light, bowed to Stef as she climbed out of the car. ‘Greetings,’ he said in his cultured British accent. He made no attempt to shake Stef’s hand, but wafted his fingers through the lintel of the doorway; pixels scattered from his fingertips like fairy dust. ‘At least in European manners, this is how to announce one is only a virtual presence. I hope this suit – that’s how I think of my various bodies, as “suits” – is acceptable to you both.’

‘It’s fine,’ Penny said. She was looking steadily at Stef. Then she approached her sister, one pace, two.

Stef stood rigid, almost at attention beside her luggage, unwilling to respond. There was a stiff moment.

Penny said, ‘Here we are, in person together, for only, what, the fourth time, the fifth? Since—’

‘Since the Hatch opened.’

‘Right.’ Penny stepped back, subtly. ‘Sorry. Old habits die hard. Even after all this time. We always hugged, before.’

Earthshine watched this exchange with lively interest. ‘The “always” applies to you, Penny Kalinski. To what you remember. But to your sister Stef, the “always”, the past before the Hatch incident, did not include you at all.’

‘That’s right,’ Stef said. ‘Lucky me. Suddenly I gained a sister.’

A look crossed Penny’s face, like the passing shadow of a defunct Heroic Generation sunshield. ‘And I,’ she said, ‘feel like I lost one.’

‘Fascinating,’ murmured Earthshine. ‘Fascinating. But here we are standing in the heat. Please, come into the shade, both of you . . .’

The old building extended to several storeys and an underground extension. For Stef, the most striking feature of the ornate interior was a sweeping marble staircase down which the virtual projection of Earthshine marched with convincing footfalls, his shadow shifting in the soft lighting. ‘Once this was an Italian-owned bank,’ he said, ‘but it has been put to many other uses over the centuries. Including a bookstore, when they still had paper books. A real historical relic . . .’ Cleaning robots worked discreetly.

They reached a relatively small, cool, windowless, underground reception room, where Earthshine invited them to sit on overstuffed armchairs, and offered them water, American-style soda, coffee, from a self-service counter. Penny took a coffee, Stef a glass of water. The room was without decoration, save for a big block of what looked like sea-eroded concrete on one wall, maybe half a metre across, its deeply pitted face marked with a mesh of concentric circles and arrowing lines, apparently intentionally carved. Stef remembered a similar design on a brooch Earthshine had worn before. The peculiar item distracted her; it looked elusively like some kind of map, a schematic, but she could not have said a map of what.

‘So,’ Earthshine said, sitting with legs crossed, fingers steepled. ‘It’s good of you to have come so far to meet me – and to take a break from your work schedules, which I know is a sacrifice for both of you. Thank you too for agreeing to put up with each other’s company, at least for a short while. Welcome to my underground lair! Or one of them.’ He smiled, with a show of apparently charming self-deprecation. ‘That’s how you think of us, isn’t it? Terrible old monsters, ruling the world from our furtive dens.’

Stef said, ‘I like to think we’re a bit more sophisticated than that.’

But Penny countered, ‘No, you got it about right.’

Earthshine grinned. ‘You contradict each other. In your talk, even in your choice of drinks. Whatever one does, the other must not follow. How fascinating. And yet by behaving this way you become ever more the mirror images that you each appear to reject . . .’

For Stef all this was picking at a scab. She snapped, ‘Is there a point to this?’

‘Oh, indeed there is,’ Earthshine said. ‘In fact your oddly coupled nature is what I primarily wish to talk to you about. I have followed your trajectories since that strange day on Mercury when the Hatch was opened. Well – you won’t be surprised to learn that. It’s what you would expect of me, isn’t it? To watch over you all, like some inquisitive god.’ He leaned forward. ‘I have asked you here, you see, because I have learned something. I have found something.’