THEATRE Love and Understanding Bush Theatre, London

'I wish I had a friend like me," proclaims Richie in Joe Penhall's new play Love and Understanding. It's sad to say that this is a minority sentiment. Richie is to friendship what nemesis is to a relaxing holiday. Charismatically portrayed by Paul Bettany, he's a decadent, self-deceived drifter, who has spent the best part of his twenties bumming around the world in search of instant kicks and pretending to be a brooding Graham Greene creation.

Now this tall, elegantly squalid, demonically irresponsible figure has landed out of the blue on the doorstep of Nicolas Tennant's very funny Neal, his closest friend and temperamental and physical opposite - a small, stocky, chronically-overworked, permanently uptight hospital doctor. The last thing Neal needs at this juncture is a difficult lodger or a reminder of less pressurised times. His relationship with his live-in girlfriend Rachel (Celia Robertson) is going through a sticky patch, since she, too, is a stressed medic and they work different shifts. She's not the only on-site temptation for the intruder; there's also Neal's professional supply of drugs, which Richie is soon contentedly raiding.

Penhall came to public attention as a dramatist of considerable promise with Some Voices, the much-praised 1994 play about a schizophrenic released into the mad streets of Shepherd's Bush. If Pale Horse - his next piece tracing the descent into the inferno of a disintegrating sarf-London widower - represented a consolidation of his talents rather than an extension of his range, then Love and Understanding feels, to me, like a step backwards. It is directed, with great style, by Mike Bradwell on a milkily translucent set and the proceedings raised a lot of loud laughter from the super-keen first-night audience. But I found myself hard put to believe in either the initial situation or how it develops.

Why doesn't Neal fling his friend out the moment he embarks on his devious psychological manoeuvrings? Richie would have to have some very deep hold over him (guilt? attraction?) to prevent this obvious course, but though the play insists that they have been close friends since childhood, this feels highly unlikely, given that Richie's manner and accent shriek public school wastrel while Neal, with his worried ways and Wolverhampton accent, is all anxious upward mobility. Then again, Rachel and Neal would have to be morons rather than educated medics to fall for Richie's transparently disingenuous efforts to manipulate them apart.

"People like him are having the fun. Even when he's not having fun, he does it better than me," complains Neal, who, approaching 30, feels he squandered his life passing exams and spending 100-hour working weeks "pushing shit uphill".

Disillusion with conditions in the health service certainly informs Neal's behaviour and it was amusing to note, at Friday's performance, that after his strict strictures on rationalisation and privatisation, the script had been brought bang up to the minute. "You'll be blaming the last government, next" was the twinklingly emended response and you wonder how many new plays are, at this very moment, undergoing hasty revision.

But it is a mark of this play's curious artificiality that neither Neal's professional disenchantment nor Rachel's romantic nostalgia for carefree student days can adequately explain why they allow a walking anti-advertisement for hedonism to lure them into a temporary hell of half-hearted unfaithfulness, ditched careers and whisky-slurping regression.

Booking to 31 May (0181-743 3388) Paul Taylor