They open a door and enter a world.

Most of the novel is set in Narnia, a land of talking animals and mythical creatures that is ruled by the evil White Witch. Four English children are relocated to a large, old country house following a wartime evacuation. The youngest, Lucy, visits Narnia three times via the magic of a wardrobe in a spare room. Lucy's three siblings are with her on her third visit to Narnia. In Narnia, the siblings seem fit to fulfill an old prophecy and find themselves adventuring to save Narnia and their own lives. 

The Lion, the Witch and the Wardrobe was ranked ninth on the BBC's The Big Read poll.  Time magazine included the novel in its list of the 100 Best Young-Adult Books of All Time,  as well as its list of the 100 best English-language novels published since 1923.