Mydidae, Soho Theatre, London

Yes, even the toilet will be in use on stage during Mydidae at the Soho Theatre

The DryWrite company likes to set its authors intriguing challenges. For example, to test the degree to which it is possible to simulate the authentic voice, it once gave a group of dramatists the choice of submitting either a genuine or a comprehensively faked piece of verbatim theatre, with the audience left to guess which were the trustworthy ones.

Now, pursuing a fascination with the subject of intimacy and privacy, it has issued BAFTA-winning writer, Jack Thorne, with the brief of creating a play located entirely in a bathroom.

The result is Mydidae, a two-hander unveiled now on a fully-plumbed set (by Amy Jane Cook) in Vicky Jones's powerfully acted production in the confined studio-space of Soho Theatre Upstairs.

As the play follows a young couple (Marian and David) through the course of a day – from morning ablutions through to a horribly failed attempt at a candle-lit romantic bath in the evening and its chastened aftermath – we gradually pick up that this is the anniversary of a devastating event.

Thorne eloquently uses the perspective of the bathroom, a place where people in a relationship will happily and unguardedly perform their bodily functions in front of each other, to throw into relief those things that can only be shared with excruciating pain in a union that has come to be held together by guilt and despair as much as by love.

Starting with the couple's bantering rituals over the flossing and the peeing (he pees; she flushes) and embracing some droll observation of male insecurity (he is seen fluffing his private parts in readiness for the shared bath), the play moves with a disarming stealth to a show-down that forces them to strip one another naked emotionally to the point of flaying.

In the earlier parts of the piece, I sometimes felt that the writing was a bit too sketch-show jokey and meandering and, even though it tries to justify it psychologically, there's implausibility about aspects of the pair's mutual ignorance.

Initially, too, I thought that the bathroom was the last place where Marian would wait in the last scene. But I was persuaded by the truthfulness of the script here and by the performances throughout of Phoebe Waller-Bridge, who piercingly exposes the pain and fragility that lie under Marian's gawky flippancy, and by Keir Charles who offers a lovely study of the difficulties of being a new man.

To December 22; 0207 478 0100