A crusty Oxford don, author of books on Milton and morality, approaches his 50th birthday. He has never married, has no children. He sits in his college rooms and writes a story about a group of evacuees who are billeted with a crusty old professor.

Fifty-five years later the story has sold 95 million copies in 41 languages and is the blockbuster film for the Christmas holidays. So what magic did CS Lewis weave into his classic story, The Lion, the Witch and the Wardrobe?

Primarily, it’s an imaginative treat. Lewis said the stories began “with seeing pictures in my head”. This included one that became the first magical encounter of the story: “a picture of a faun carrying an umbrella and parcels in a snowy wood”, a picture he held in his mind from his teenage years.

That image characterises another of the book’s qualities, a messy magic in which a being from Greek mythology can carry an umbrella. It is a quality summed up in the book’s very title, with its two vibrant nouns and a mundane piece of furniture. And it is a quality that irked Lewis’s friend JRR Tolkien, who advocated a greater separation of worlds in fantasy, as exhibited in The Lord of the Rings.

The children entering the land of Narnia encounter a world of fauns and giants but also of lamp-posts and teapots, a world in the grip of an evil witch.

The portrayal of evil provides another strand of the book’s magic. Lewis dwelt on the complexities of morality and the most striking portrayal of villainy, for my money, is not the cold and cruel character of the witch, but the more interesting character of Edmund, one of the children.

While Lewis’s dated attitudes may rouse criticism, his portrayal of Edmund’s slide into evil and betrayal is recounted with real understanding.

He becomes trapped in a web from which the escape proves costly, when Aslan the lion tackles the witch’s magic.

The book is often misunderstood as an allegory - a coded presentation of the death of Christ. It is inevitable that aspects of Lewis’s faith will inform his writing - that’s what faith does to life - but he rejected the idea that this was an allegorical reworking of the gospels. It’s deeper than that. The story taps into Lewis’s belief in how the universe works.

That belief is woven into a great story, well worth revisiting. Just try the opening chapters and you’ll see why this book has resisted the half-death in which many children’s classics slumber. When the children need to hide from the housekeeper and climb into the wardrobe, you’ll see how this story keeps weaving its magic.

Huw Thomas is a headteacher in Sheffield. The Chronicles of Narnia: The Lion, the Witch and the Wardrobe is due to be released on December 8