His Dark Materials, Olivier Theatre London

It has been flagged as the most ambitious piece of staging at the National in 20 years. It certainly attempts the impossible - the storytelling equivalent of trying to pour all the oceans of the universe into a swimming pool or embodying a multi-dimensional, quantum physics conception of the cosmos in good old-fashioned 3-D.

So the stakes were already sky-high. The tension was then screwed up a notch when insuperable technical difficulties caused the cancellation of three previews and the postponement of the press day of this epic two-play dramatisation by Nicholas Wright of Philip Pullman's best-selling His Dark Materials trilogy.

As a result, public attention has naturally focused on the event as a will-they-pull-it-off-or-won't-they feat of technical wizardry. But watching Nick Hytner's six-hour production on Saturday, I found that it wasn't the parts where the Olivier Theatre was showing off its astonishing resources (with the mighty drum-revolve working overtime to dredge up diverse worlds) that made the keenest impact on me. The bits that I shall treasure are the moments of heart-stopping simplicity where Wright, Hytner and the two superb central performers (Anna Maxwell Martin and Dominic Cooper) prove they have grasped the emotional core of this inspiring saga - a colossally arduous rite of passage into puberty in which a pair of 12-year-olds from parallel universes redemptively re-enact the Fall, their positive embrace of sexual love and adult consciousness liberating Creation from the life-denying repression of the wicked Christian church.

The planned movie version (scripted by Tom Stoppard) will have major problems casting the leads, if the idea is to take child actors (à la Harry Potter) through a quasi-realistic progression into maturity, film by film. Wright's stage treatment, by contrast, makes a splendid narrative virtue out of using young adult performers in the roles and out of the fact that his adaptation is delivered in one fell swoop rather than in instalments.

The main action is presented as a massive flashback, with a framing scene that introduces us to Lyra and Will, now aged about 20, meeting for one of their psychic midsummer night trysts under a tree in the Botanical Gardens. They sit side by side but are literally worlds away from each other, cruelly separated in their alternative Oxfords, their souls yearning for reunion.

From this poignant vantage point, it feels utterly natural that the same performers should show us how Will and Lyra became these people and I have never seen impersonations of the tricky pre-adolescent state so uncondescending or empathetic. Maxwell Martin radiates the tough, weary street wisdom and underlying vulnerability of an illegitimate girl abandoned to her own devices by neglectful, high-born parents. Though the adaptation gives short shrift to his background story of protecting a schizophrenic mother from government harassment, Cooper movingly suggests the damaged side of Will that the boy's courage manages to surmount.

When this pair come to apply to university, their CVs will run to the length of, well, three-decker novels and include such extra-curricular activities as "cutting into parallel universes with a magic knife" and "witnessing the voluntary death of God". As it whisks you through their cosmic quest (which ranges from the Arctic wastes to the Land of the Dead), the production feels over-hectic with plot at the expense of allowing you to savour the weird poetry of these extreme situations. A huge tilted mirror, swirling dry ice, and grey-garbed ghostly prisoners successfully evoke the nightmare neverendingness and insubstantiality of death's kingdom, rendered more unnerving by the broken-winged flap of the excellent, shrieking harpies. But the haunting de Chirico-like quality of Cittagazze, the sun-soaked Mediterranean harbour town wiped clean of adults by the spectres and left to scavenging children, barely has time to register in the onward rush.

In Lyra's world, human beings have visible daemons, animal emblems that incarnate the person's soul or subconscious. With moulded wire heads and trailing wispy bodies of swishing translucent cloth, some of these puppets (designed by Michael Curry of Lion King fame) create the requisite impression of airy spiritual guardianship; others, though, seem to enjoy about as dynamic a relationship with their human sidekick as a fox fur stole does with its wearer. The best moment comes when Samuel Barnett, who manipulates Pantalaimon, Lyra's daemon, takes off his black mask and eerily reveals himself as her other constant personal companion, Death. That's the kind of boldly imaginative, non-technology-reliant touch that one would like more of in a production that too often (as in its depiction of the cutting into other worlds) leaves you wondering how Robert Lepage or Theatre de Complicite might better have handled things.

The stage version has more wit than the novels (it's a droll stroke that the dastardly cleric, the President of the Consistorial Court, has been made a close cousin of Donald Rumsfeld), but the questing horn motif that accompanies each appearance of the Subtle Knife suggests that it would take an opera to do justice to their density. Patricia Hodge is terrific as Lyra's devious glamourpuss of a mother.

A shame, then, that Timothy Dalton plays her super-intelligent but emotionally narrow father, Lord Asriel, as the hearty games teacher from hell. It's the wonderful younger pair who carry the evening - the ardour of their love and the painfulness of the fate that requires them to return to their parallel worlds making a conclusion as piercingly sad and noble as any I have seen in the theatre.