The Lion, the Witch and the Wardrobe

Four children, Peter, Susan, Edmund and Lucy Pevensie, are sent into the country to live with Professor Kirke because of the air-raids on wartime London. On their first morning, it pours with rain and they play hide-and-seek indoors. Lucy scrambles inside an old wardrobe full of coats, only to find:
... that what was rubbing against her face and hands was no longer soft fur but something hard and rough and even prickly.
"Why, it is just like the branches of trees!" exclaimed Lucy. And then she saw that there was a light ahead of her; not a few inches away from where the back of the wardrobe ought to have been, but a long way off. Something cold and soft was falling on her. A moment later she found that she was standing in the middle of a wood at night-time with snow under her feet and snowflakes falling through the air ...
Lucy is in Narnia, but the country is in the grip of the White Witch's spells and it is always winter. The Witch had come into Narnia many years before, and there is only one way in which her evil magic can be overthrown. It has been foretold that her reign will end when two Sons of Adam and two Daughters of Eve sit on the four thrones at the castle of Cair Paravel. So the Witch has made sure that anyone who sees a human in Narnia will tell her straight away - or be turned into stone.
Lucy narrowly escapes being trapped, and slips back through the wardrobe, scared, excited, and not a little bewildered to find that her adventures have taken no time at all in her own world. Not surprisingly, her brothers and sisters don't believe her story, especially as the wardrobe remains obstinately a wardrobe! Narnian magic is not like a light switch: you cannot turn it off and on at will.
However, the children do go back into Narnia when they least expect it and find everything just as Lucy described - only perhaps more real and more frightening. The kindly Mr. and Mrs. Beaver keep them safe from the White Witch's secret police - but there is a traitor in their midst and the four children soon discover that only Aslan can protect them from the terrifying danger that threatens to destroy them all ...