Lane's house looks as sterile as, hopefully, the hospital in which she practises as a consultant. Everything is white - walls, floor, sofa, and even Lane herself, who is dressed from head to foot in white. Employed to keep everything immaculate is Matilde, a Brazilian cleaner. The only trouble is, Matilde doesn't like cleaning, and wants to become a comedian - constant is her search for The Perfect Joke.

Lane's sister Virginia offers to help - she has no objection to cleaning, but she likes a bit of dirt too: "My sister doesn't know whether her husband is sleeping with a prostitute or not," she remarks sagely, "because she does not smell his dirty underwear." Suspicions are aroused, however, without the need for any sniffing - in among the family washing are a couple of pairs of saucy red briefs, which are most certainly not Lane's. Her husband is a surgeon, and he does indeed confess to having an affair - but with Ana, a 67-year-old patient who is suffering from terminal cancer.

Sarah Ruhl's comedy The Clean House was originally produced in the US in 2006. It is sharply and beautifully written, without a superfluous word anywhere. The laughs come thick and fast from the moment the curtain goes up, but you also soon become aware that the characters are being developed beyond the normal comedy stereotypes. The reason for this becomes apparent in the second half, when the humour continues, but Ruhl adds a very moving layer of poignancy as Ana begins to succumb to her illness.

The Royal has assembled a top-notch cast for this in-house production. Patricia Hodge presents a Lane who is plainly stressed out at work. Yet you feel that underneath she is a very interesting and humorous person. Natalia Tena is wonderfully quirky and feisty as Matilde, while Joanna McCallum is the perfect, cheerful foil as Lane's sister. Eleanor Bron is a vivid and moving Ana, while Oliver Cotton provides a colourful Charles, Lane's errant husband. Confidently directed by John Dove, every aspect of this show is first-rate. It's a long time since I've seen anything better.

The Clean House continues at Royal & Derngate, Northampton, until February 16. Tickets: 01604 624811 or www.royalandderngate.co.uk The production tours to the Oxford Playhouse from March 3-8.