The word Brexit didn’t even exist when John Lanchester published his last novel, Capital, early in 2012. It’s hard to imagine a more vivid expression of the decade’s darkening mood than the contrast between that novel, a warm, comic-realist panorama of post-crash London, and his new one, set in a dystopian Britain of national service and slavery where the border policy has recently shifted from the already draconian-sounding one in, one out to a simple shoot-to-kill of incomers, or ‘Others’, assuming their boats haven’t already been sunk by coastguards or drones.

We see all this through the eyes of young Kavanagh, serving his compulsory two-year turn patrolling the titular, 10,000-kilometre wall. Officially known as the National Coastal Defence Structure, it was built in the wake of a climate-related crisis known as the Change, which has left large parts of the planet underwater.

As one of 300,000 so-called Defenders, he’s stationed on the north Devon coast, serving twelve-hour shifts, two weeks on, two weeks off. He disarmingly announces that ‘there isn’t much narrative … much of the time, what you’re mainly looking at is concrete.’ The novel’s energy comes initially from descriptions of how dire things have become. Early on Kavanagh says:

Until about ten years ago, Others who showed they had valuable skills could stay, at the cost of exchanging places with the Defenders who had failed to keep them out. The law was changed because this fact became known to Others and started to act as a ‘pull factor’, a reason they came here. Now, today, Others who get over the Wall have to choose between being euthanised, becoming Help or being put back to sea.

Any children of Others who somehow make it through are forcibly separated from their parents but are eligible for citizenship-by-microchip (what’s happened to the blue passport, Lanchester doesn’t say), though only to serve as ‘Help’, state-operated slavery by another name. Among the more sinister aspects of Lanchester’s society is how readily people have adopted its dehumanising taxonomy. It’s a rare moment when Kavanagh breaks with the prevailing incuriosity about life beyond the Wall to ask one Help if there’s a word for ‘what we call the Change, in your language?’

‘Coo-ee-shee-a,’ I thought he said. I didn’t know if I’d heard that correctly and had no idea what it meant, but there was something in his eyes that stopped me from asking more.

When Kavanagh looks it up on his ‘communicator’ to find it’s Kuishia, Swahili for ‘the ending’, it’s a reminder that problems elsewhere in the world extend beyond the lamented loss of beach holidays and unseasonal produce. For Kavanagh and his fellow Defenders it chiefly means having to guard against a new wave of refugees who have begun to arrive, a politician warns, ‘in numbers, like the numbers from many years ago when the Change first struck. Big numbers, dangerous numbers.’

After much scene-setting involving freezing-cold vigils, training exercises and descriptions of post-Change cuisine, Lanchester introduces explosive action with a plot concerning Kavanagh’s gruff captain, a one-time Other nursing a secret scheme. There’s also romance of sorts in the shape of Hifa, a fellow Defender who suggests having kids (Breeders become exempt from service because of the need to breed Defenders).

But it’s part of the novel’s poker-faced affect to ensure its drama isn’t character-led. We learn little about Kavanagh, other than that he’s from ‘the Midlands’ and, in common with the rest of his generation, is resentful of his parents’ pre-Change, pre-Wall life. He once took their car ‘without permission, got drunk, overrode the autopilot, slid off the road and hit a tree and trashed the battery’. You sense that his status as a representative of a screwed-over generation is what most interests Lanchester. Ultimately you could see all this as a more vivid way of confronting the intergenerational injustice that he has written about before in his celebrated nonfiction work on the economy. We’re more than halfway through the book before a line of dialogue lets slip that Kavanagh’s first name is Joseph, underlining his role as a blankly Kafkaesque fall guy just in time for him to end up a victim of the system he’s enforcing.

As the novel segues into a survivalist sea story that never quite lives up to the promise of the first two-thirds, it hits you that Lanchester has more or less inverted the premise of last year’s Booker-shortlisted Exit West, Mohsin Hamid’s speculative fantasy about borderless migration, in which black portals mysteriously open up around the globe. The Wall is the bleaker book, yet it’s infinitely less solemn, in part because of its chatty, pithy voice, recognisable from Lanchester’s journalism. ‘When bullets come close, the noise they make as they go past changes from a zing to a crack,’ Kavanagh says, sounding very like his author. While Lanchester’s familiar, informative tone didn’t always meet the demands of Capital, a multi-character ensemble piece, it’s an asset here, in part precisely because it’s comically at odds with what’s being described. But look again at the paragraph quoted above: to whom exactly is Kavanagh explaining how his world works? It’s a question that recurs as the novel progresses. Lanchester provides a solution, folded into an elegant sign-off, but it feels a bit of a cop-out in view of the grim future he’s outlined. Then again, if The Wall is even half right, maybe artistic quandaries are going to be the only type we can reasonably expect novelists to solve.