The story follows the adventures of two young children, Polly and Digory, as they're tricked by Digory's magician uncle into entering another world. Actually, where they end up is more of a gateway to many worlds. The children inadvertently awake an evil witch-queen who has destroyed all life in her world. The witch follows the children home to London, and in their attempt to return her to her home, they end up in Narnia.
A nice treat for readers is that they get to witness the creation of Narnia from the very beginning.What's interesting is that the genesis of Narnia is somewhat similar to the genesis of Tolkien's Middle Earth: they both start with a song. In Narnia it is Aslan and the stars who start the song (with Aslan finishing it). Readers also get an explanation of why some animals in Narnia can talk but others cannot.
Digory and Polly must work to combat the evil they have brought with them into Narnia, which of course is not an easy task. And of course they don't entirely succeed, as you might notice Jadis is doing quite well when Lucy, Peter, Susan and Edmund arrive in Narnia. You have to read the book to find out why that is.
If you're a fan of any of the Narnia books (or movies) you definitely need to read The Magician's Nephew. For those of you who are waiting for the movie adaptation, you might have to wait a long time, because those involved in the creative process have disclosed that the different shareholders involved in the Narnia movie franchise are in a deadlock about what movie to make next and when.