In which book do the Pevensie children first enter Narnia?

The Pevensie children first enter Narnia in The Lion, the Witch and the Wardrobe. Lucy discovers the magical world when she steps through the back of a wardrobe while exploring an old country house.

The four siblings—Peter, Susan, Edmund, and Lucy—are staying with Professor Kirke during the Second World War. Lucy's first visit initially seems unbelievable to the others, but Edmund later follows her through the wardrobe. Eventually all four children enter Narnia together and become central figures in the struggle against the White Witch.

At the time of their arrival, Narnia is trapped in a winter that has lasted for a century. The children meet Mr. Tumnus, encounter the White Witch's rule, and later join Aslan, the great lion who leads the fight to restore Narnia.

A frequent mix-up comes from reading the books in chronological order. The Magician's Nephew explains Narnia's creation and is set earlier in the world's history, but it was published later. The Pevensies' first entrance occurs in The Lion, the Witch and the Wardrobe, the first Narnia story Lewis published.