Saeed and Nadia meet at an evening class in an unnamed, presumably Middle Eastern city “swollen by refugees” but not yet “openly at war”. Saeed is “an independent-minded, grown man, unmarried, with a decent post and a good education”. Nadia is less straightforward-seeming: she doesn’t pray but wears a “conservative and virtually all‑concealing black robe”, works in an insurance company but rides a “scuffed-up hundred-ish cc trail bike”, has veered off from her parents and lives alone. Saeed quickly falls in love with her. Nadia, to begin with at least, is “not certain exactly what she was feeling, but was certain it had force”.

In previous novels, Hamid has used a heavily inflected narrative voice to filter everything through a personality that is not his own, but which he nevertheless owns as the author. In The Reluctant Fundamentalist we eavesdrop on a marvellously well-sustained dramatic monologue that reveals a great deal about its speaker, while also concealing precisely what he intends to do to his listener. In How to Get Filthy Rich in Rising Asia, Hamid ingeniously adapts the form of a self-help book to create a tale-telling “you” who becomes intimately realised while remaining a nameless everyman. Exit West confidently adopts yet another kind of voice – a tone of radical simplicity that in the opening 50-odd pages borders on brutality, and makes every conversation, every detail, every scene feel at once vital and under threat.

Predictably enough, this is most obvious in scenes of outright violence as militants close in on their city prey. Nadia’s doctor cousin is “blown to bits, literally to bits, the largest of which … were a head and two-thirds of an arm”. The man who sells Nadia and Saeed magic mushrooms is beheaded, then “strung up by one ankle from an electricity pylon where [his body] swayed legs akimbo until the shoelaces his executioners used instead of rope rotted and gave way”. Saeed’s own mother dies while looking in her car for a lost earring, “a stray heavy-calibre round passing through the windscreen … and taking with it a quarter of her head”.

A major part of Hamid's achievement is to show how profoundly social damage injures private lives

The mixture of clarity and restraint in such passages is very impressive, and confirms Hamid’s reputation as a brilliant ventriloquist who is deeply engaged with the most pressing issues of our time. It is also interestingly at odds with the device he then uses to connect his story with its remaining sections. Faced with imminent catastrophe, Saeed and Nadia escape from their city through a “black door” – one of several that briefly opens among the accumulating ruins. Although we have already been alerted to the existence of these doors – by rumours that our hero and heroine overhear, and also by passages that Hamid interpolates into his text, in which figures suddenly emerge from nowhere in places ranging from Australia to Marrakesh – they still seem remarkable. They maintain an element of magical strangeness opposed to the plainness of the prose in which they are presented, and lead us to think of the novel as a form of parable.

And so it is – a parable of hideous contemporary familiarity and strangeness. Once Saeed and Nadia have passed through their door, they find themselves in a refugee camp on the Greek island of Mykonos, where in the process of feeling variously relieved, frightened, outraged and threatened, they plunge more and more deeply into the questions of identity and nationhood that dominate the remainder of the book. “In this group [on the island],” Hamid says, “everyone was foreign, and so, in a sense, no one was” – preparing us for an ideal of integration that his characters find variously attractive and difficult to achieve. When they leave the island by passing through another door to Germany, and then another that leads them to London, they enter a city that is rapidly “filling up with [the] tents and rough shelters” of other refugees, where every form of homogeneity is perceived to be under threat.

Hamid describes these threats in terms that deliberately echo some of the intolerant voices raised by Brexiters: there is a “reclaim Britain for Britain” movement of “nativists”, for instance, which soon forces a political crisis. And not only political. A major part of Hamid’s achievement in Exit West is to show how profoundly social damage will injure private lives – not only in obvious ways (physical injury, homelessness), but by hampering the ability to construct any sort of life outside their sphere of influence. As Saeed and Nadia try to develop their own true selves, external pressures accentuate their different attitudes to sex, to worship, to how they view their homeland. At the same time, rootlessness causes the most widespread possible lack of integration, by provoking refugees from similar backgrounds to identify and shelter with others like themselves, rather than continue to live in a melting pot.

Facebook Twitter Pinterest Radically simplified style … Mohsin Hamid. Photograph: Sarah Lee/The Guardian

Saeed and Nadia fear that they are about to be engulfed in a second wave of carnage, and the ferocity of the opening scenes in Exit West will make every reader share their anxiety. But by this stage in the novel Hamid has in fact changed tack somewhat. He is now less interested in showing violence than in describing a solution to the problems it embodies. The military backs down. “Decency on this occasion [wins] out,” he tells us, “and bravery, for courage is demanded not to attack when afraid.”

As Saeed and Nadia develop their own selves, external pressures accentuate their different attitudes to sex, to worship

While this saves Saeed and Nadia from battle harm, it cannot undo the hurt already inflicted. As they begin working with other displaced people in a vast series of camps on the outskirts of London known as the “London Halo”, the distance that has opened between them only increases. In the process, Hamid’s simple style acquires an almost fairytale quality, as he continues to explore the extent to which personal lives are subordinate to political circumstances: “Every time a couple moves,” he says, “they begin … to see each other differently, for personalities are not a single immutable colour … So it was with Saeed and Nadia, who found themselves changed in each other’s eyes in this new place.”

The narrative depends on clear storytelling so much for its effect, and Hamid makes this clarity a means of generating suspense. So suffice it to say that while Saeed and Nadia continue to try to prove their individuality in ways that are not entirely dictated by war and its consequences, they also continue to function as a means for Hamid to think about questions of national identity and social cohesion. This makes for a comparatively quiet conclusion to a book that began with fire and blood.

When he approaches his conclusion by asserting that “We are all migrants through time”, and when he shows Saeed feeling that “it might be possible, in the face of death, to believe in humanity’s potential for building a better world”, his bare-statement style works against him. Initially it compelled us to sup full of horrors. Now it seems a little thin, and therefore conveys a sense of wishful thinking. Perhaps this is always a risk when writers use the same style to dream of utopia after toiling through a dystopia. But it nevertheless means that we exit Exit West admiring its depiction of nightmare more deeply than we feel persuaded by its description of a bright future – no matter how much we might sympathise with the principles defining that future.

• Andrew Motion’s latest collection is Peace Talks (Faber). Exit West is published by Hamish Hamilton. To order a copy for £9.99 (RRP £14.99) go to bookshop.theguardian.com or call 0330 333 6846. Free UK p&p over £10, online orders only. Phone orders min p&p of £1.99.