Polygamy is a pleasant but fragile state, according to this play from young writer Matt Charman.

Based around a suburban ménage à cinq, it has a thoughtfulness and wit that are involving but is an unexpected insight or flash of inspiration short of fascinating.

Its strength lies in the way Charman makes real the strange network of support and mutual reliance holding together the multilateral family at its centre. Creating a likeable man to run a voluntary harem is not easy, yet Larry Lamb's Maurice Pinder is such a man. He exudes a calming, authoritative presence, a passionate devotion to family and an infinite ability to see the beauty in compromise.

To barren first wife Esther (Sorcha Cusack), he offers motherhood; to flighty second wife Fay, a free nursery for her child while she indulges her rampant libido. To the much younger Lydia and Rowena, he provides sanctuary while they get their lives together.

It is eerie at first how at peace the women are with each other and with Fay's son, Vincent, a nervous, over-loving child played with a strange, runtish feyness by Adam Gillen. This delicate ecology is ruined, first by Jason, a one-night stand of Fay's who begins to harass the household, then by new wife Irene, brought in by Maurice to take control of the situation.

Clare Holman is a bad girl you can cheer as recalcitrant fortysomething Fay; sexy and confused early on, her drunken grabs at freedom are the highlights of a second half that takes too much time to reach its inevitable conclusion.

For Charman, having spent the first half explaining Maurice's philosophy, runs out of things to say. The introduction of Tessa Peake-Jones's hilarious agent provocateur Irene seems to announce that he is looking for an ending.

Sarah Franckcom's directorial choices - slow, sad set-changes and twinkling, contemplative music - numb the senses and encroach upon the consistently good jokes that are the play's best quality. Still, interesting stuff.

Until 27 August (020 7452 3000).