Avery is an exceptional child. Everything he does is precise, from the way he washes his face in the morning, to the way he completes his homework -- without complaint, without fuss, without prompt. Zib is also an exceptional child, because all children are, in their own way. But where everything Avery does and is can be measured, nothing Zib does can possibly be predicted, except for the fact that she can always be relied upon to be unpredictable. They live on the same street. They live in different worlds. On an unplanned detour from home to school one morning, Avery and Zib find themselves climbing over a stone wall into the Up and Under -- an impossible land filled with mystery, adventure and the strangest creatures. And they must find themselves and each other if they are to also find their way out and back to their own lives.