Having never read the book, I watched this film with no knowledge of anything Narnian. Having watched the film though, I know all there is to know.
Four children stumble across a magical land called Narnia at the back of a wardrobe. When they get there, they find themselves having to fight and protect Narnia from the White Witch. Edmund (the youngest boy) falls for the White Witch's charms and gets captured. Trapped, he has to get rescued by his three other siblings.
After rescuing Edmund, the four children have to escape the chasing White Wtich, with the help of two friendly beavers (who are so cute by the way). After stumbling across Aslan, the almighty Lion, they engage in a huge duel between good and evil to save Narnia.
The visual effects are genius and the story is totally captivating. A really enjoyable film and one that is watchable again and again.