‘You know I’m not really a gadget kind of guy. But this is pretty wicked.’

Rod winced. ‘“Wicked”? How old did you say you were, Dad? Hold on to your hat, the take-off is kind of sudden.’

With a hum of a biofuel engine and a subdued roar of jets, the craft jolted forward across a grassy sward, bumping a little on the uneven ground. There was nothing like a runway at Reboot; there weren’t enough aerial visitors to justify it – and most of them came in airships that didn’t need a runway at all. Evidently this little plane didn’t need a runway either. After a remarkably short taxi, it leapt into the dark sky.

They didn’t step immediately. Rod had the plane bank on autopilot in a wide, lazy circle as he checked the Stepper box at his waist, and then opened up a small pack of pharmaceuticals and began to guzzle pills. As far as stepping was concerned Rod had mixed ancestry: his father, Joshua, was the world’s prototype natural stepper, but there were phobics on his mother’s side – those unable to step at all, such as his notorious uncle, whose name he’d taken. Rod himself was somewhere near typical. With a Stepper box, Rod could step maybe three or four times a minute, but he’d be hit by nausea each time, and needed treatments to control the reaction. Luckily for him, by the time he was trying to fulfil his boyhood dream of flying the twains – the great stepwise-bound freight-carrying airships – the anti-nausea drugs had reached a pinnacle of effectiveness, and steps coming every few seconds, or even faster, were manageable.

This self-medication went unremarked by Joshua. Although he did wonder if the modern treatments still turned your piss blue.

The cabin windows were big and generous, and as the ground opened up beneath him Joshua was able to see the scattered lights of Reboot, and the neighbouring farms and shepherds’ shelters. But they hadn’t risen far before the settlement was lost in the continent-spanning forest, a deep green-black sea on this moonless night. ‘Makes you think how few we are, on worlds like this, even after all these years. And after all the breeding we’ve done.’

Rod grunted. ‘To me and my buddies this is normal. A planet’s not supposed to glow in the dark.’ He stowed away the pharma kit. ‘So you ready?’

‘Let’s go.’

Rod tapped a corner of another of his glowing screens.

The first step was a faint jolt, like a bump in the road – and suddenly they were in a rainstorm – and out of it with the second step.

After that, worlds flapped past Joshua’s view, one after another, variations on a theme of black, with not a light to be seen on the ground below. From the beginning the stepping was faster than Joshua’s heartbeat, which was a little disconcerting, like too-fast music. But as the step rate increased the inevitable juddering sensation soon smoothed out, to be masked, in fact, by the faint vibrations of the smooth-running engines as the plane settled into its run, heading generally geographic west, towards the heart of the continent and the footprints of Wisconsin, even as the stepping continued.

‘Nice machine,’ Joshua said.

‘Sure. “Wicked.”’

‘I won’t ask how much you paid to use it. Or how you earned the money in the first place, living the way you do.’

‘I’m making my living my own way, which you know nothing about. Look, here I am for you, just as you asked. You wanted us to spend time. Fine. This ride is my gift to you, Dad. Happy birthday, OK? But why the hell are you doing this?’

‘Why the hell not? I’m fifty. I’ve spent my life wandering the Long Earth. Why not cross a hundred thousand worlds in a day? Why not mark it with a stunt like this, while I still can?’

‘I hate to tell you, but I’m pretty sure it’s not a speed record.’

Joshua shrugged. ‘I don’t care about comparisons. I never much cared what other people thought of me, as long as they left me alone.’

‘Well, maybe you should care, Dad. I mean, like Mom said, you could have marked your birthday with something which wouldn’t have involved you being out here all alone.’

‘Like what? A barbecue?’ Joshua looked at his son sideways. Rod’s face was softly illuminated by the glow of the control tablets.

‘Now you sound like your mother. Or your grandfather. Once Dan was going to be a twain driver. Now here’s Rod the comber, who knows it all.’

Rod replied irritably, ‘I wanted to fly twains when I was a little boy, for God’s sake. And I got to do it, for a while. But there aren’t the opportunities now – you know that.’

It was true enough. Twains still flew locally, especially across the more industrialized Low Earths, but the big Datum–Valhalla route, a ‘Long Mississippi’ that had spanned a million worlds with a bridge of trade and cultural exchange, had withered after the Datum Earth had been effectively knocked out by Yellowstone. And then, after the catastrophic winter of 2046 and a new wave of emigration from the battered Datum, most Long Earth trade had shrunk back to relatively short-range exchanges.

Still, changing career plans was one thing; changing your name was a different kind of statement.

Joshua hesitated before saying, ‘I think it’s disturbed your mother that you’ve started to use your uncle’s name, you know.’

‘It’s my middle name. You gave it to me.’

‘True, but—’

‘This is the hidden secret of the Greens, isn’t it? Jack the great political firebrand, my mother the midwife: once hero trekkers, now the heartbeat of Reboot. But they are only out here because they abandoned their phobic son back on Earth, and look what happened to him in the end.’

After Helen’s eighteen-year-old homealone brother had played a part in the anti-stepper terrorist nuking of Datum Madison, Wisconsin, he’d spent a lifetime in custodial institutions. He’d died in there only recently, of an infection he’d caught in hospital. Joshua realized with a shock that he’d committed his single, terrible crime when younger than Rod was now.

‘OK, but you’re kind of throwing this in their faces. Jack’s particularly. You shouldn’t judge them, Rod. They just couldn’t find a way to make it work for everybody.’

‘We all make mistakes, eh, Dad?’

‘Yes, frankly. You just haven’t made yours yet, son. Or maybe you just don’t know it.’

‘Thanks for that. Now maybe you should shut up and let me fly this thing.’

‘Rod, I—’

‘Forget it.’

After that, much of the night was spent in not very companionable silence. Long hours of darkness which Joshua spent much of beating himself up for what he’d said, or hadn’t said, not for the first time where his family was concerned.

Maybe Joshua slept a little. He suspected Rod napped too, leaving the flying to the autopilot. Even the flaring of an occasional Joker did not disturb them.

The sun came up on all the worlds of the Long Earth.

Some fifty thousand steps from the Datum they were still deep in the thick band of worlds known as the Mine Belt: cooler and less well forested than the Earths of the Corn Belt, which began around the location of Reboot and stretched away to the stepwise West. The Mine Belt worlds were mostly exploited only as sources of minerals of various kinds, either for local use or for export to the Datum and the Low Earths – though even that kind of trade was dwindling now. But there were herds of animals to be seen, drawn to the water courses and lakes, mostly four-legged mammals but not much like anything that populated the human cultural imagination: things like giant camels, and things like elephants with oddly shaped tusks, stalked by things like huge cats. As they stepped, the herds, dark flowing masses, were there and gone in an eye blink.

They had a silent breakfast of Helen’s cookies and lukewarm coffee from a flask.

Around eight a.m. the character of the worlds below changed again, subtly. This was the Ice Belt, a band of periodically glaciated worlds, of which Datum Earth – at least in its primordial state, before humanity got to work – appeared to have been a typical example. These Earths were cooler, with open prairie and grasslands, the forests shrunk back to patches of evergreen, and tundra in the far north. As Joshua had learned during his own forays into the Long Earth, and on that first journey of exploration with Lobsang two decades ago, when you crossed the Long Earth it was like flying through the branches of some tremendous tree of possibilities and probabilities. The closer you got to the Datum, the more links in the chain of coin-toss cosmic accidents that had led to the peculiar circumstances of the home world locked into place, and the more familiar the landscapes became. So now, on the sparsely populated grasslands below, they saw animals of the kinds alongside which humans had evolved, even if said animals hadn’t necessarily survived to feature in the modern world: mastodons and mammoths, deer and bison. In most of these Earths the epochal collision of the Americas, North and South, must have taken place, for they saw immigrants from the south, such as giant sloths and armadillos the size of small cars.