Edinburgh Festival LANARK The Assembly Rooms

The line that gets the biggest laugh in TAG's theatrical version of Alasdair Gray's Lanark comes when young, asthma-plagued hero Duncan Thaw is asked by his working-class Glaswegian father what he wants to be when he leaves school. A teacher, maybe, or an engineer, or a doctor? Quick as a flash, and with all the scowling humourlessness of adolescence, Tom Smith's fine Duncan replies: "I want to write a modern Divine Comedy with illustrations in the style of William Blake." How long, a parent might wonder in the circumstances, until the child starts believing he's Napoleon?

In fact, with Lanark Gray himself achieved just such an ambition - a novelistic Divine Comedy replete not just with illustrations but with a split-level hero (Thaw/Lanark); shifts between a Fifties Glasgow twinned with its apocalyptic equivalent, Unthank; typographical bizarreries designed to show that the book is a physical object produced by an economic system; a dislocated chronology; and a portrait of Scotland as a phantasmagoric microcosm of the world. As easy, you might feel, to land Concorde in a heliport as to deposit that lot on stage.

Alastair Cording's adaptation, directed now by Tony Graham, does not entirely disabuse you of this prejudice. The book begins with more than a hundred pages of Lanark's travails in the sunless, diseased city of Unthank and in the creepy, cannibalistic institute to which he escapes. Only then, via an oracle, does he learn of his previous identity as Duncan Thaw. In the stage version, by contrast, the hero starts remembering his Glaswegian past almost immediately, allowing the production to alternate between present and past.

The consequent juxtapositions do indeed help to underline briskly the correspondences between Smith's Duncan and Laurance Rudic's woeful-faced Lanark. Both pine for love, suffer skin disease because of its absence, and seek redemption in art and Utopian politics.

What gets lost in this oscillating arrangement, though, is not just the fact that Lanark's amnesia is a key component of the misery and dread he suffers in his first sojourn in Unthank. Also badly blurred is any sense that Lanark is an evolutionary development of Duncan. Matters aren't helped by the addition of yet another doppleganger in the shape of the Painter (Stuart MacIntyre). Accompanied by a string trio and singing leaden lines like "Look at the lightning over Sinai/ It must be moved/ Two and a quarter inches to the right", the Painter is supposed to represent Duncan's artistic creativity, help us to visualise through music the progress of his great Creation mural, and offer (according to the programme) "a unifying point for the strange double identity of Lanark and Duncan Thaw". In practice, though, you may feel that he results in a piece that is over-egged with alter egos.

The cast are highly resourceful, even if only some of the doubling is meaningful (in a memory-stirring, Wizard of Oz-like way). The sub-operatic music sounds arty rather than artistic and a person who had never been to Glasgow would get no impression of its texture from this abstemiously designed piece. An honourable, at times moving, failure, it makes you more rather than less keen to see Gray's cinematic version make it to the screen.