Jack Thorne's intriguingly titled two-hander (named after a group of large flies, apparently) is set entirely in a bathroom, where a couple spend a day going through familiar routines and sharing some startlingly intimate moments.

I've always been fond of plays that evoke lifestyle (think Stoppard's Real Thing), and Thorne - aided by designer Amy Jane Cook - does it exceptionally well here. Marian and David's clawfoot bath tells you everything you need to know about the kind of people they are; metropolitan, creative, the archetypal young and upwardly-mobile middle class couple.

But both bear scars that come into frightening perspective as the play progresses. The centrepiece scene, in which the couple share a bath, builds to a shattering climax and inserts brutality into what should be the most intimate of acts.

Indeed, 'intimate' is the watchword. Stage nudity can often seem an unnecessary distraction, but here it feels essential and is skilfully choreographed by director Vicky Jones; the first time David disrobes he starts playing with his genitals in the mirror, an action that injects humour where there could be embarrassment (especially in the tight confines of Trafalgar Studios 2).

Phoebe Waller-Bridge (who I first saw in Thorne's 2nd May 1997 at the Bush) and Keir Charles have built up great chemistry since the play premiered at Soho Theatre last year and now seem to be playing with the text, testing its edges. Something that comes across sharply is the subtle but crucial class war being waged between the privately-educated Marian and the football-loving everyman David.

And when David's masculine crisis reaches its brutal head, Charles effectively demonstrates how domestic violence can spring from the most unexpected of places (there are shuddering echoes of recent events in South Africa).

The key to the couple's conflict is only revealed at the end, and when it comes it seems almost unnecessary. There are enough everyday factors at play - financial worries, stifled ambitions, insecurities - to make this dark day in the bathroom all-too understandable. Thorne has long been a writer to watch, and with Mydidae he takes his place at the top table. Superb.