Elling is one of those rare, precious pieces of spoken-word drama that persuades even sour old cynics like me to go home riding high on waves of joie de vivre. There is no better theatrical tonic in town.

Elling shows how two thirty- something patients in a Norwegian mental asylum - brought to memorable comic life by John Simm and Adrian Bower - manage to survive when released. There is not a shred of sentimentality or mockery of the mentally disturbed in Paul Miller's fluent production.

Having been assigned a flat, the duo are warned by their social worker that they must prove they can live unsupervised. It seems unlikely.

Sim's eponymous, mother-fixated Elling, first seen hiding in a hospital cupboard, and later facing up to domestic life in top-buttoned raincoat and occasional pyjamas, wears a remote look of prissy disdain and scribbles obsessively in a little black note-book.

Flitting between sanity and loopiness the pair make their own mad, comic logic. "Mother did all the shopping. I was in charge of ideology," Elling remarks to Adrian Bower's fine, unbright, randy Bjarne, whose woolly hat signals his strangeness. "Why have an apartment if you go out?" he asks Bjarne.

What gives the play its poignancy is the way in which against all the odds, and the problems posed by their oddness, the symbiotic pair survive. Luck and a fine mutual concern save them.

Elling is picked up in non-sexual fashion and befriended by the widowed Alfons, once a major poet, but now drifting around in affable serenity and bonhomie, a state of mind beautifully conveyed by Jonathan Cecil.

"I have made a friend on my own and without the aid of the Norwegian government," Elling exclaims in pride as he begins to envision himself as a poet, too.

Meanwhile, Bjarne falls for the pregnant, older woman upstairs, thereby inducing jealousy in his friend, though not sufficiently to threaten their attachment. By the end both men have recovered enough to be capable of coping with the world.

Disgracefully, the programme says nothing about Ingvar Ambjørnsen, who wrote Elling as a novel, or its stage adaptors, Axel Hellstenius and Petter Naess. Simon Bent is prominently credited for a further adaptation, but I suspect chief credit for this elating, dark comedy should go to its originator.

• Until 26 May (020 7610 4224).