Midwinter, Soho Theatre, London

It is 10 years into a bitter war. The people in the besieged city are starving. A woman drags in a stolen horse and proceeds to guzzle its rotting flesh. Oh God, you're probably thinking, it's yet another Howard Barker. But no, this is a new play by Zinnie Harris, the young dramatist who made such a strong impression with Further than the Furthest Thing, a piece that beautifully created a strange, self-consistent world, as it examined the fate of the dispossessed and linguistically isolated islanders of Tristan da Cunha .

Directed by the playwright with a touch that is as delicate and astringent as her writing, Midwinter is located in a less tethered, more mythical landscape. An old man and his mute eight-year-old grandson enter while the woman is eating. Without food, the boy will die, though if he can only hang on for another two years, the state will feed him well, for 10 is now the age of conscription. The woman strikes a deal with the grandfather. The child can have the horse to consume, if she can have the child to rear. Desperate, the old man consents, only to discover that the woman intends to change the boy's name, and to pass him off as her little son who had died, when her husband returns after a decade from the now-concluded war.

What follows feels like an inversion of the French legend of "Martin Guerre". Here, the person who may be an impostor is not the returning warrior - a man racked not only by mental demons from his time fighting at the front, but tormented by actual parasites in his eyes, which not only make him suffer the periodic sensation that his eyes are being flooded with lemon juice, but will probably end up blinding him.

No, the person whose identity causes ripples of concern here is the wife, Maud, or should that be "Maud''. She is brought to dubious, abrasive, but deeply affecting life here in an astonishing performance by Ruth Gemmell that manages to show the character as downbeat, driven, tough, tender, full of animal cunning and humanly incautious.

The boy cannot (or will not?) speak. The father wants to do father-son things and try to make up for all that lost time. But the army has left him with the worst of several legacies. One is to be disqualified, through ingrained violence, from bringing up a child. To break the couple's hold on his grandson, the grandfather (played with a lovely, sensitive watchfulness by John Normington) begins to sow the seeds of suspicion.

Without giving too much away, it might be said that one is reminded of that Greek heroine's revenge with the burning shirt of Nessus. Except that there is nothing long-distance about Maud's eventual retaliation. And that the end is gently, tentatively heartening.

To 19 March (0870 429 6883)