Theatres don't generally like people writing about the first preview of their shows, particularly when the production is of an ambitious new play. But when the National Theatre is concerned, the production standards are so high that nobody who sees a preview is likely to be short-changed. At last night's performance of this stunning new play by NT veteran Tony Harrison, the only minor glitch was the failure of the two leading actors to make it up out of the bowels of the stage revolve to take their curtain call. Fram means 'forward' in Norwegian and it was the name of the polar exploration vessel which Fridtjof Nansen designed and built so that it would not be crushed in the Arctic ice. Harrison's play is loosely based on Nansen's life, but it's less an exploration of the man's pyschology than an extended meditation on the themes and issues that dominated his career. That makes it sound a bit dry; it isn't. The play is visually a feast, so much so that I went away remembering the images, not the words. Harrison's rhyming verse ranges from the sublime to the panto-level banal. Aurora rhymes with snorer, discussions rhymes with Russians. At times his quirky in-jokes about poetry,the National Theatre and Greek tragedy (his forte) verge on the brink of being self-indulgent, but all this is redeemed by some superb scenes where his language has a lacerating sharpness. The set by Bob Crowley, who is listed as co-director with Harrison, eclipses anything I've seen on the Olivier stage for its ambition and invention, ranging from Westminster Abbey to the Arctic ice via the Bolshoi Theatre and making imaginative use of slides and back-projection. It's constantly surprising, with moments where the author illustrates his theme of humanity and inhumanity with disconnected tragic images, including two African children frozen to death as stowaways in the wheel housing of an airliner, and a Kurdish poet with his mouth and eyes sewn up. Harrison gives us not just the gilded boxes and the red curtain and the music of the Bolshoi, but the real ballet as well , danced by the Royal Ballet's Viviana Durante. She's worth the ticket price on her own. Her appearance is peripheral to the theme of the play, but it adds to the extraordinary quality of this production. That wonderful actress Sian Thomas, whom I last saw playing a Holocaust denier in a ghastly play at the Hampstead Theatre, has a great part in this one -- the actress Sybil Thorndike -- and makes the most of it. The highlight is a scene where she intervenes in a rather lifeless discussion about whether photography or art can best convey the horror of the Russian famine of 1922 by acting out the drama of a starving woman who starts to eat human flesh. It's an old-fashioned tour de force. Harrison builds the drama around two pairs of characters, the first being Sybil Thorndike and her friend the classicist Gilbert Murray (also portrayed in fictional form in Shaw's Major Barbara at the National). Their link with Nansen is that like him, they were early supporters of the League of Nations. Harrison uses them to explore whether art can ever be a force for good. Can putting on a play ever help the starving? Or should we accept Adorno's grim conclusion, quoted in the programme, that writing poetry after Auschwitz is barbaric? Eventually Murray (a delightful performance by Jeff Rawle) smashes the mask of tragedy which is one of the play's recurring images. The other twinned characters are Nansen and his polar companion Hjalmar Johansen, who later took to drink and killed himself. As Nansen evolves from egotistical Darwinian to selfless humanitarian, Johansen (Mark Addy) accompanies him like a cynical doppelgaenger. Johansen suggests that if Darwin is right, then surely we should forget our taboos about cannibalism and put each other in the stewpot. The play ends on a more upbeat note, with the suggestion that morality and generosity can trump Darwin, and humanity can still cooperate to save itself. Nansen believed that the weakening power of the sun would eventually push the earth into a polar freeze. A century later when we're all worried about global warming, the scientific picture looks quite different, but the moral question is exactly the same. We have to help each other or die. While Johansen is suitably Mephistophelian and has some of the best lines in the play, Nansen the worthy Norwegian comes over as a crashing bore, which gives Jasper Britton who plays him an almost impossible task. He's a cypher without much inner life, and it seems to me that Harrison rather lost interest in him at an early stage. This isn't a biographical play about character, however, but a work of images and ideas. There are some minor characters (the ARA relief workers in Russia and the do-gooders Ruth Fry and Eglantyne Jebb) who seem to me inessential to the play, and there's a long scene in a London drawing room at the end of the first half which needs cutting. But I would rather go to see an ambitious non-realist play like this one than a dozen run-of-the-mill realist plays by writers who don't have Harrison's powers of imagination.