Così, King's Head, London

Slipped in among the musical repertoire of the OperaUpClose season in Upper Street comes this wacky, semi-autobiographical 1992 Australian play, set in 1971, by Louis Nowra, about patients in a lunatic asylum putting on Mozart's Così Fan Tutte – without the music. It's also about the madness of putting on a show in the first place, the terror of performing and the treachery of backstage relationships.

"We're actors, we're the opposite of people," says the Player in Stoppard's Rosencrantz and Guildenstern, and there's an extra twist here with a cast of non-professional social outcasts behaving much more like actors than people.

Pill-popping Zac (Cameron Harris), deprived of his piano duties in favour of an offstage recording, wants to supplant Mozart's overture with a blast of Wagner on his accordion, and he's got the tunic and helmet horns to match. And scary pyromaniac Doug (Nathan Lang) has escaped the closed ward to have sex with, well, anyone, really.

Nowra based the play on personal experience, and ends it, movingly, with a summary of what happened to the inmates he worked with. He, too, is put under pressure as the director Lewis (Matthew Burton), who is assailed for directing a musical comedy while the students outside, and his fellow director Nick (Hamish MacDougall), who happens to have stolen his girlfriend, are protesting against the Vietnam war.

The director, Adam Spreadbury-Maher, fills in the scene changes with bursts of the Velvet Underground, the Rolling Stones and George Harrison, so the show feels like a real King's Head throwback – all the way to Robert Patrick's Kennedy's Children, with their damaged illusions and personal tragedies.

Cherry Truluck's design is cheap and cheerful, and the acting is crude mixed with dire. But the show warms up as Shelley Lang as the junkie hairdresser and Sophie Brabenec as a withdrawn depressive find their feet and their way around the stage.

Da Ponte's libretto is cleverly inflected through the show, too, with Maggie Daniels's pig-tailed Cherry – aka "Despina" – wildly overstepping her interventionist romantic brief before all come on stage to recite the final "number", just as effective, in this context, as it usually is with the sublime music.

To 13 July (020 7478 0160)