The 39 Steps, Tricycle Theatre, Kilburn, London

In his capacity as one half of the brilliantly funny National Theatre of Brent, Patrick Barlow is used to presenting stage treatments of epic books and ambitious topics - everything from The Bible to The Complete History of Sex - with just a pair of performers. So he must feel that it's almost scandalously spendthrift to be lavishing a company of four actors on the theatrical version he has concocted (from an original concept by Simon Corble and Nobby Dimon) of The 39 Steps.

John Buchan's spy thriller has been a magnet for movie directors. But though the story (as mediated by Hitchcock) may begin in a music hall and end full-circle at the London Palladium with the crucial "Memory Man" routine, it's not surprising that the theatre has given it a wide berth. It's precisely the lunacy of trying to translate a roving, suspenseful chase caper to the stage that seems to have appealed to the makers of this show, which is given a fast-paced, resourceful and attractively tongue-in-cheek production by Maria Aitken.

The result is like watching Hitchcock's classic 1935 film, as ruined by the adroitly hapless National Theatre of Brent.

I have mixed feelings, though, about the value of the piece. It is, without doubt, drolly ingenious. The famous pursuit across the Highland moors is evoked in potty shadow-play, replete with biplane. The hero dangles from a horizontal ladder to suggest his near fatal suspension from the Forth Bridge. Charles Edwards offers an expert and endearingly affectionate spoof of a stiff-upper-lipped clubman as Richard Hannay, the toff who finds himself circumstantially framed for murder and then pitched into a plot to foil the plans of a German spy-ring, and save vital Air Force secrets.

Catherine McCormack is almost equally winning in a trio of female roles, particularly as the cool blonde alibi who takes off her wet nylons while handcuffed to Hannay, with comically awkward and sexy consequences.

Above all, Rupert Degas and Simon Gregor are a joy as a daft double-act who surface wherever our hero goes, playing - with lightning switches of wigs and props - all the other characters, from underwear salesmen to an ill-matched crofter couple.

The problem is that I think much of this talent is misapplied. Hitchcock's movie achieves the fine art of combining wit and suspense but there's precious little of the latter in this stage show, which is too busy sending itself up in an artful combination of dexterity and pretend am-dram incompetence. Patrick Barlow's style of comedy works best when he adapts and punctures pieces that take themselves very seriously but Hitchcock's take on The 39 Steps renders this approach redundant, because of its own in-built, knowing humour.