Theatre / DANTI-DAN Hampstead Theatre, London

"This is the kind of place," says one of the characters in Danti- Dan, "where if you lost your virginity, someone would find it and bring it home to your mum." But "losing" virginity sounds altogether too inadvertent an activity to describe the determination to dump hers shown by Cactus (Sophie Flannery), the 13-year-old girl whose raging hormones fuel Gina Moxley's often very funny new play.

Brought to Hampstead in a winning Rough Magic production by Lynne Parker, the drama is set in an Irish backwater 10 miles from Cork in the summer of 1970, and if you ever needed further proof that a repressed society creates sexual obsessives, this play would be your man. The preoccupations of four of the five young characters rarely venture above the navel - except, perhaps, at such moments as when 14-year-old Dolores (Eileen Walsh) contemplates the lovebites on the stomach of Ber (Dawn Bradfield), her 16-year-old sister, and exclaims "Oh God, it looks like shingles."

Some of the best comic moments in the play, as well as its eventual tilt into tragedy, derive from the way it shows how adolescents of roughly the same age can be at wildly different stages of sexual development. Almost malevolent with frustration, Cactus insists on locking Dolores in fierce snogs, but Dolores, while keen in a goofy way on hunting for the dirty bits in books ("Take me, Richard... take me to the hospital") can't see what all the fuss is about. This kind of discrepancy is demonstrated in extreme form by the title character, played by Alan King, a podgy, bespectacled 14-year-old with a mental age of eight who rides around in a cowboy outfit on an invisible horse and winds up the victim in Cactus's own brand of "poker".

What gives the drama life is the quality of its observation ("One of the charms on my bracelet got caught in his zip"), beautifully served by actors who are only a few years older than the characters they portray.

As Cactus, Flannery has an alarming power. Ostracised at the end, she remains defiantly unrepentant, even to the extent of asking whether Dolors managed to get a feel of Dan's willy before the rather predictable fatal accident. It's hard to know whether the play is saying that Cactus was always going to turn into this kind of girl (in which case her earlier frustration would be less representative) or that the accident hardens her in what would otherwise have just been a passing phase. Whichever way you look at it, much of Danti-Dan is an impure delight.

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