Here's the tiny family that always ends up in king-sized trouble! Years ago, Grandpa Little set off to explore and was never seen again and the mystery of his disappearance was never solved. But now, Lucy and Tom have stumbled across a secret room where Grandpa kept many papers and maps. And they think they can figure out the path he took into the wilderness the last time he set out on an expedition! The family packs up and heads upstream. Surprises await them around every bend: huge turtles, river rapids, and a new family of tiny people! But when their odyssey is over, will the Littles know any more about what happened to Grandpa Little?