Mother Clap's Molly House, National Theatre, London

Dangerous yet delightful enterprise deserves to drum up a roaring trade

"I tell you, poufs have got it sorted!" cries the envious straight man who finds himself at a chic and increasingly rampant gay sex party at the top of the second act of Mother Clap's Molly House. Hang on, this is the Royal National Theatre ... Well, no one can say you weren't warned. After all, the publicity image featured a hunky lad in nothing but a corset showing off his bare bum. Yet the only shocking thing about the evening is how splendidly enjoyable it is.

It starts with a bang as the all-knowing chorus sings an introduction, just one of the brief, pointed songs that frame the scenes. "Enterprise, it makes you human/ Business shapes our heart and hand." The link between making love and making money comes as no surprise when you consider that the writer is Mark Ravenhill, who sprang to fame with Shopping and F***ing. But that is about the only predictable thing about a play that explores pleasure in both the past and present.

Back in 1726, trade is dwindling at Tull's tally shop, which hires dresses out on a daily rate, mostly to the whores of London.

But when Tull swiftly dies of the pox, the cowed and nervous Mrs Tull has to decide what to do with the business. Her apprentice, however is out wandering and discovering his sexuality.

Together they realise that there's money to be made and eventually they are making a fortune in a Molly House – one of the real-life gay brothels of 18th-century London from which the play takes its name – with their journey contrasted by the sexual mores of their counterparts in a similarly highly sexed present day.

The show is not so much laced with bawdy humour as steeped in it, but instead of relying on "naughty" suggestiveness to crank up the laughs, Ravenhill and the director, Nicholas Hytner, paradoxically create startlingly innocent comedy from refreshingly frank language and staging.

Mother Clap's has a level of purely theatrical ambition missing in nearly all the National Theatre's recent new plays. But the realisation of that ambition is down to Hytner's marvellously animated production.

It is peppered with boisterous individual performances and whole scenes are brought to three-dimensional life on Giles Cadle's atmospheric, garret-like set with winning vigour and flair.

The evening, however, belongs to Deborah Findlay. Her Mother Clap begins in timidity and terror and grows into a hilarious performance of high comic brilliance which never loses sight of truth and tenderness.

Like Caryl Churchill's Serious Money, Mark Ravenhill's play exposes the delights and dangers of commerce, but this constantly surprising evening deserves to do a roaring trade.