Us/Them, National Theatre

Unimaginable tragedy is given poignant, piquant form in Us/Them. The hour-long performance piece from Belgian theatre company BRONKS has arrived at the National after a much-acclaimed Edinburgh Festival premiere last year.

In its intricate weave of frontline semi-reportage and slyly subversive comedy, Dutch-born writer-director Carly Wijs allows a sense of play to inform at every turn this highly physical account of the Beslan school siege in September, 2004. The terrorist act propelled a little-known town in the Russian Caucasus into grievous front-page news. 

The conceit here is that two young adults play as many children caught up in the three-day atrocity that saw the hostage-taking of 1100 or so people, 334 of whom died - children making up more than half the number of casualties. From a set of statistics too grim for contemplation has come a shape-shifting, even ticklish piece that suggests what trauma might look like as it is both experienced and recollected from the inside looking out.

As the accomplished and appealing Gytha Parmentier and Roman Van Houtven construct an elaborate cat's cradle (pictured below) extending the length and breadth of the Dorfman stage, one clocks the gently comic detail that lends proceedings an often mordantly funny air even as the gathering horror of events exerts its vice-like grip. Van Houtven's tussle with the shirt he is loathe to take off is just one of various moments that surprises in its ability to prompt a smile. 

At first, the performers use chalk to mark out their given terrain, so as to help us visualise the locale. But barely has Beslan been identified as a town of 33,646 where "nothing much happens" - shades here of Thornton Wilder's Our Town - before the seizure of the school by Chechen rebels changes everything, not least the children's relationship to such newly hostile surroundings. Whether battling dehydration or doing their best not to have to pee, the unnamed pair talk over or across each other on some occasions and turn eerily silent on others. Wijs strikes a fine balance between mania and stillness whereby the characters' pointed one-upmanship - whose maths is more correct? - only lands because the performers work so well as a team. (The more numerate in the audience will have a field day.) 

Reality jostles with flights of escapism and fantasy, accompanied at one point by the Mission Impossible theme tune, the threat of annihilation to be found in our glimpse of an ominous foot on the detonator just in case. Words are spoken ("paedophile", for one) whose full meaning these characters clearly don't know, while political point-scoring and emotional indulgence are dispensed with so that the children's unvarnished candour and savvy can carry the day. 

At no point does one find the cutesy condescension and play-acting that often comes when adults impersonate kids, and only the final sequence with its pile-up of possible endings strikes too glib a note. The title, meanwhile, implies a set of binary opposites that Us/Them neatly overturns: by the curtain call, the play has worked on the imaginative reach of its audience to ensure that such divisions at the grimmest extremes of life may just find an antidote in art. 

• Us/Them at National Theatre/Dorfman until 18 February.
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