The Unbelievers, Royal Court Theatre - grimly compelling, powerfully performed

Change, we're often told, is the engine of drama: people end up somewhere markedly different from where they began. So the first thing to be said about Nick Payne's blistering new play The Unbelievers is that its concept is as brave as leading lady Nicola Walker's take-no-prisoners performance.

Playing a mum who can't stop obsessing about the disappearance of her son (as who could given such a situation), Walker makes something ferocious out of human fixation, grabbing Payne's narrative by the throat and allowing Marianne Elliott's correspondingly unflinching production to strike boldly at the heart. 

Even in less accomplished form, this play would be part of the continual uptick in the Court's fortunes that has seen Giant transfer to the West End (Broadway follows next year), whilst their imminent upstairs entry, Porn Play, comes with its own A-list, transfer-worthy talent attached. But playing a woman whose absent son further fractures her relations with those family members very much in her midst, Walker careers about Bunny Christie's suitably clinical set - a recessed area suggests a waiting room of sorts - as a parent who will never find peace. "His body used to be my body," she says late on of the absent Oscar, whose disappearance defies common sense: Might it have something to do with the braces he recently had fitted? Bullying? Who will ever know?

Real-life equivalents to this situation are easy to find, whether one thinks of Etan Patz in New York back in 1979 or, of course, the ceaseless mystery that has surrounded the story of Madeleine McCann from 2007 onwards. Payne's genius is to translate Miriam's monomania into a splintered narrative, as if her own brain were scrambling for a linearity that events have rendered impossible. The result shows us goings-on in the first week of this teenager's vanishing, and also one and seven years later, Miriam's intensity set against two ex-husbands and as many daughters. Each relation must confront a woman who in some essential way is newly lost to the person they once knew: presence, we discover, can itself be an absence.

Miriam is the play's feral core, the others orbiting around her unyielding drive and purpose as one might expect from the author of the widely traveled Constellations. That play's movement towards death comes with its own prismatic structure, though The Unbelievers feels weightier, more ambitious in its endeavour to seeks some kind of structure when life throws meaninglessness your way.

Working here on a far larger canvas than with the earlier play, Payne manages to make the supporting roles sing, whether in one or another character's advocacy for a seance or a memorial or simply the value in faith found in ex-husband Karl (Martin Marquez, splendid), a vicar who feels "pretty good" about heaven. (The play's ability to treat religion seriously, without derision, is one of its many strengths.) 

Elliott, the director, is in full control here in a play that can be seen as the inverse of her breakout success years ago with The Curious Incident of the Dog in the Night-Time, which was about a young man in search of his missing mum (played originally, as it happens, by Walker). You very quickly sense a contrast between the daughters, whether that be the pregnant Margaret (Ella Lily Hyland)  pondering her mother's sanity or older sibling Nancy (the expert Alby Baldwin, pictured above), who ends up on a date with Mia (Isabel Adomakoh Young) - one of the many parties enmeshed in the investgation. 

Lest this sound all unrelieved gloom, the moods shift as needed across the nearly two hours, no interval, and there's a brilliantly funny scene in which Mischief Theatre alum Harry Kershaw shows up as Margaret's partner, only to profess a love for the Flintstones alongside an interest in the world's declining puffin population. Kershaw is matched comic beat-for-beat in that scene by the superb Lucy Thackeray, playing the new partner of Miriam's ex, David (Paul Higgins), who scoops up Miriam in a much-needed embrace whilst agreeing all the while with Benjamin's musings on sand eels. That discussion proves too much for Miriam whose attention is directed towards such matters as banana skins and meatballs, but it functions as comic dynamite within the play. 

Walker throughout never once sentimentalises a character who is herself a psychically suppurating wound, with a bloodied hand at the start as a visual correlative, as if one were needed. Audiences may not warm to someone so nervy and on edge and who possesses so little interest in being liked - several near me walked out at the performance attended. But that response misses the point of a play that stares into the abyss and well beyond, demanding meaning from a cosmos that simply cannot find it. I, for one, believe in the gathering power with which The Unbelievers argues its unflinching case, and Walker's complete embrace of this role is itself a thing of wonder.

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• The Unbelievers at the Royal Court till 29 Nov