Mother Clap's Molly House, Nt Lyttelton, London

Pleasures of the flesh

"I tell you, poofs have got it sorted!" cries the envious straight man who finds himself at an increasingly rampant gay sex party in Mother Clap's Molly House. What? This is the Royal National Theatre... Well, no one can say you weren't warned – 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 sing an introduction, one of several brief, pointed songs framing 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 the only predictable thing about a play that explores pleasure in both the past and the 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 is left holding the business. The apprentice (the excellent Paul Ready), however, is tentatively exploring his sexuality, and together they discover there's money to be made out of supplying sex to a whole subculture of men. Mrs Tull becomes Mother Clap, running a molly house – based on one of the real-life male brothels of 18th-century London – with the characters' unsentimental education paralleled by highly sexed 21st-century counterparts.

Mother Clap has a level of purely theatrical ambition that has been missing from nearly all the National Theatre's recent new plays. But the hugely engaging realisation of that ambition is down to Nicholas Hytner's marvellously animated production. The storytelling is beautifully paced, partly because of his fashioning of a stream of boisterous but sincere performances – notably Danielle Tilley's hilarious, sharp-eyed and squealing country virgin who cannot wait to sell her maidenhead for an eye-popping 20 guineas.

The show is steeped in bawdy humour, but happily, instead of relying on naughty suggestiveness for laughs, Hytner and Ravenhill manage to create startlingly innocent comedy from refreshingly frank language and staging. The first half in particular carries the audience along an immensely satisfying arc of sustained energy, with whole scenes brought to three-dimensional life on Giles Cadle's garret-like set with winning vigour.

The tension flags in the second half, but even there Ravenhill holds you by voicing intriguingly unexpected opinions through characters such as William Osborne's world-weary party host, with his moments of rueful insight, or Con O'Neill's compassionate comic portrait of a pig-farmer. But the evening belongs to Deborah Findlay, whose Mother Clap grows from timidity and terror into giddy, barnstorming self-confidence. Rooted in truth, it's a star performance of comic brilliance, fired up by and glowing with the show's most attractive quality: its tenderness.

Like Serious Money – a play by Caryl Churchill, to whom Ravenhill is seriously indebted – Mother Clap's Molly House may illumine the delights and dangers of commerce, but this gloriously surprising evening deserves to do a roaring trade.