An involved and brilliantly funny performance from current flavour of the month John Simm makes Elling tick. Centred on him, a show which might so easily have been an over-long and uneventful curiosity is a delight - a pocketful of sunshine to see us through this lousy summer.

Adapted by Simon Bent from the Norwegian film, this is a play about - yippee! - the rehabilitation of the mentally ill. Elling and Kjell Bjarne, best of pals, are allowed out of the asylum to attempt a normal life in an Oslo flat.

Set the task of convincing social worker Frank (Keir Charles) of their sanity, the odd couple struggle comically on until, guided by the strength of their own friendship, they win through.

Summarised, it sounds trite - perhaps even insultingly so. In practice, it is often gorgeously uplifting. As Bjarne, Adrian Bower is a lofty simpleton with a heart the size of a Buick and an unsatisfied libido to match.

Elling is the brains to Bjarne's brawn: an over-grown mummy's boy - a devious, chronically cerebral wreck of a man. Simm, star of BBC series Life On Mars, is wholly inhabited by the role.

His body taut, language mannered, he marries the scuttling walk and camp mannerisms of John Inman with the weak, confused pomposity of Iain Duncan Smith. Moreover he exudes more warmth and exhibits better timing than either of those comedy icons. It is a total performance, a self-transformation so complete that when Simm relaxes for the curtain call, it is as if some malevolent spirit has finally abandoned his frame.

Through him, we get an impression of the difficulties our heroes face in the most everyday things. His Tintin-ish features bulge with the effort of answering the phone. The prospect of leaving his flat patently terrifies him.

With the bar set so low, there's disproportionate joy to be had from the couple's small victories: an exchange of presents at Christmas, a visit to a café, a piss-up.

Which is just as well, because the play turns out to be a long succession of such victories. The episodic plot - featuring Alfons, Elling's wizard-like friend and mentor, and Reidun, the girl who at first sight loves and is loved by Bjarne - has a naïve air of fairy tale.

Such tales are generally kept short - this play runs for two and a half hours. There's good feeling enough to last the whole way, but it's a shame to test it.

Until 6 October (0870 060 6632).
www.theambassadors.com/trafalgarstudios