Lucy Pevensie's siblings, Peter, Susan, and Edmund, don't know what to think when their sister tells them that she has found a whole new world inside a wardrobe. At first, they think she is just making up a story for fun, and when they go back to look into the wardrobe, there is no sign of snow or trees or anything else. It is just a plain wardrobe, and Peter tells Lucy that she has made a “jolly good hoax.”

Lucy, however, sticks to her story and feels miserable that her siblings don't believe her. Then Edmund finds out that Lucy has been telling the truth, for he, too, enters Narnia through the wardrobe. However, when Lucy once again insists that the world in the wardrobe is real, Edmund does not back her up. He says that Lucy is merely pretending, even though he knows he is lying. Lucy is devastated.

Peter and Susan decide they need help with the situation, and they go to the professor with Lucy's story. He listens closely and then asks, “How do you know that your sister's story is not true?” Susan responds that Edmund said they were only pretending. Then the professor asks whether Lucy or Edmund is generally more truthful. Peter admits that Lucy has always been.

The professor then reminds Peter and Susan that accusing someone who has always been truthful of lying is a very serious thing. He also assures them that Lucy is not crazy; he knows this well by looking at her and talking to her. Then he invites them to use their logic. If Lucy does not lie and is not crazy, then she must be telling the truth, as fantastic as that may seem.

Peter then asks why they couldn't get into the other world through the wardrobe when they looked, and Susan wonders why Lucy was gone for no time at all when she said she had been away for hours. The professor tells them that their looking for the other world and not finding it proves nothing and that the detail about time actually makes Lucy's story more likely to be true. Lucy would have hidden for a while if she were really trying to fool her siblings. What's more, another world would have a time all its own. The professor concludes by telling them that nothing “is more probable” than the existence of other worlds and that they should try minding their own business.