I READ a new fairy story this week - by which I mean new to me. Before Christmas, we went to see The Light Princess, a musical by Tori Amos at the National Theatre, and was intrigued to learn that it was based on an original fairy tale by George MacDonald. The changes that have been made to the story offer an instructive primer to the moral worldview of our times.

MacDonald, who wrote in the middle of the 19th century, influenced writers as diverse asJ. R. R. Tolkien, Lewis Carroll, C. S. Lewis, G. K. Chesterton, and E. Nesbit. He was brought up a Calvinist, but, although he became a Congregationalist minister, he rejected notions of predestination and substitutionary atonement. Christ came, in his view, to save people from their sins, not from some divine retribution for our wrongdoing. And he had a decidedly uncalvinist attachment to myth.

In his tale, a king fails to invite his sister, an embittered witch, to his daughter's christening. So she puts a curse upon the child: that she will not obey the laws of gravity (apart, it transpires, from when she is swimming in the lake outside her palace). She is saved - after tribulations, which include the drying up of her beloved waters - by a prince who agrees to sacrifice his life to restore the lake.

The National Theatre version abandons the self-sacrifice, and introduces eco-themes to the desiccation of the lake, and a social dimension in a war between the kingdoms of the prince and princess, driven by the couple's intransigent fathers.

MacDonald's style is dry, understated, and shot through with the irony that characterises the best children's writing, where the text speaks to adults and children on different levels, and thus keeps both amused. His weightlessness is a metaphor for a lack of moral gravity in a princess who is unable to take anything seriously, laughs at everything, but cannot cry, or properly smile. Only when she experiences anguish does she become fully human.

Samuel Adamson, who wrote the musical's libretto, replaces the Christian notions of service and duty with a Californian me-generation exhortation to self-belief and self-esteem. He turns the story into a feminist eco-parable. But he also replaces the light irony with a heavy-handed earnestness, which hits the audience over the head with witless lecturing xenophobia, climate change, male chauvinism, and outdated attitudes to marriage and motherhood.

We hate poetry, said Keats, that has a palpable design on us. The same is true of pantos. What both religion and fairy tale understand is the importance of wonder, the potency of archetypes, and the repulsion of evil. Modern fairy stories can do that. Yet, although the blurb for the musical describes it as a "dark fairytale about grief, rebellion and the power of love", it has none of the lurking sexuality of Angela Carter, or the Freudian nastiness of Sondheim's Into the Woods. It offers only a Disney-like "Hakuna matata" sunny optimism, in which misguided self-interest is as evil as the world gets.

There is magic in the show; but it comes from the breathtakingly inventive staging (by Marianne Elliott, who also co-directed War Horse), in which four "invisible" climbing acrobats convey the princess's weightlessness. But the po-faced preachy modernisation of the fairy story's themes reveals just how at sea our modern secular culture is with the business of deeper symbolic meaning.