The "fall" into the other world happens in stages in The Lion, the Witch, and the Wardrobe. First Lucy finds her way through the wardrobe and meets Mr. Tumnus. When she tries to show Peter, Susan, and Edmund the way to Narnia, though, it has disappeared. Some time later, Lucy and Edmund both get there, but again the gateway has disappeared by the time Peter and Susan are there to check it out. Finally all four children are transported together, and the adventure can really begin.
At first, the children seem to be having little mini-adventures, meeting a mythical creature here and an impressive Queen there. This world has talking animals, sentient trees, and an everlasting winter. It's weird and wonderful.
When the White Witch hears about their presence, the four children are suddenly in grave danger. They are thrust right into the center of prophecy and politics, becoming figures of hope for the people of Narnia – but their lives are in peril!
Although all four children can tell that something is going terribly wrong, they don't know that Aslan is planning to give his own life in place of Edmund's. When Lucy and Susan witness Aslan's death, it seems like everything is lost.
Unlike other "Voyage and Return" stories, this one doesn't mix the "Thrilling Escape" with the "Return." The Pevensie children are able to triumph over evil without running away back to their own world. Instead, they enjoy a long period of happiness and prosperity in Narnia, before one day finding themselves back in England. They have lived whole lifetimes in Narnia, growing up and becoming adults, and now they are suddenly children again. Have they really changed during their experience? Well, you'll have to read the next book, Prince Caspian, to find out!