Betty, Vaudeville Theatre, London

Talk about beginning with the climax. We're in the kitchen of middle-aged Catholic spinster Betty Buchanan, who is perched atop her washing machine. Well, you can guess what – and who – is coming. Betty recalls how, on her 49th birthday, an abnormally violent spin-cycle introduced her to the raptures of orgasm, and she obliges us with a demonstration. In her sensible pleated skirt and stout sandals, Geraldine McNulty's engaging Betty becomes a juddering, shrieking turbine of autoerotic abandon.

I'm happy to report that after this one uncharacteristic incident, Betty kneels down, makes a sincere act of contrition and resumes an untroubled devotion to the faith of her fathers. Oh, all right, just kidding. Karen McLachlan's one-woman play is, to be sure, not a piece you'd want to watch in the company of Ann Widdecombe. The guilt-racked, pious virgin can't get enough of those good vibrations. Putting up little shows of girlish resistance, she starts to treat the machine as a demon lover and is always ready with an excuse ("I do need to wash my non-fast coloureds") for trying out new positions. But much as she would like to be, Betty is not a person who can take this situation sitting down. She resolves, therefore, to find a religious cure for her secret shame and is sent on pilgrimage to Iona.

This reviewer is a comprehensively lapsed Catholic and a keen aficionado of smut and there were, indeed, several moments when I laughed out loud. Even I, though, quickly came to feel that the play's iconoclasm is pretty puerile. Betty seeks help in the confessional where, predictably, the priest is a pervert who demands that she pass her moist knickers through the grille ("These pants are covered in the secretions of sin") and then buries his face in them. Just as unsurprisingly, the trip to Iona is more like sex tourism than a pilgrimage. There are some enjoyably daft details here. Reviving from a fainting fit at the orgy convened by her host family, Betty hears a strange sucking sound "like high tide licking up the wooden posts of an old pier". Our heroine is now so orgasmic that it only takes a tuning fork struck for a group rendition of "To Be A Pilgrim" to set her off.

But the relentless presentation of the faithful as just so many sex-mad hypocrites or masturbatory obsessives (the saintly Theresa is an orgiastic flagellant) grows wearing. In the interests of extending material sufficient for a sketch into a string of innuendoes lasting 75 minutes, the play is prepared to belittle everybody – Betty included. Despite a radiantly winning performance from Ms McNulty, this solo character comes across as a convenient construct – a pious figure who is somehow in on the dirty joke. Her speech is a contrived mix of biblical echo and a nudging suggestiveness worthy of Dame Edna ("I dipped into the moistness of my last wash," she reveals, arm stealing through the porthole).

For all its attempts at varying the mood, Kathy Burke's production can't disguise the over-stretched, one-joke nature of the piece. Nor can the uplifting footage of giant waves that is back-projected on to the scene where Betty is finally liberated from her religious guilt stop you from feeling that the heroine's return to a solitary relationship with a domestic appliance is a rather sad and lonely form of triumph. The piece began life in Edinburgh, and would obviously work better presented as a late-night show for a well-lubricated audience. In the West End, this shallow variant on a "vagina monologue" fails to touch the spot.

To 28 September (020-7836 9987)