Book 2 in The Chronicles of Narnia series by C.S. Lewis.
In this tale symbolic of the gospel, during World War II, four children, Peter, Susan, Edmund, and Lucy, are sent to stay at the country home of an old professor. During a game of hide-and-seek, Lucy steps into a wardrobe, only there seems to be no end to it. Just when she thinks she's found the back she steps out into a wintry forest - and the land of Narnia!
There she meets a Faun named Tumnus who tells her how the White Witch has used her magic to make it always winter in Narnia, but never Christmas. They will all discover the land of Narnia and meet Aslan, the Great Lion and true ruler of Narnia.
Though Aslan's return brings hope to the Narnians, that hope is crushed when Aslan gives himself in sacrifice to save Edmund from the Witch. Who will save Narnia now?