The Magician's Nephew by C.S. Lewis

The Magician's Nephew by C.S. Lewis is a delightful fantasy story about the beginnings of the travels between our world and the world of Narnia. Polly and Diggory, two friends in late 1800s London, find themselves unexpectedly on an adventure to other worlds ("not other planets; they're part of our world and you could get to them if you went far enough") including Charn (where they meet the evil queen Jadis, and Narnia as it is just beginning.

Unfortunately, they drag along a few other people and a horse, as well. With Jadis now loose in Narnia, what is Diggory to do? He needs to do something, since he woke her up in Charn (deliberately) and brought her to Narnia (not deliberately). Fortunately, Aslan has a solution. But will Diggory, with Polly's help be able to accomplish it?