Eight-year-old Jack and his little sister, Annie, are playing in the woods when they find a mysterious tree house full of books. But these are no ordinary books ...And this is no ordinary tree house ...Jack and Annie travel back in time, wearing only their swimsuits! When the tree house lands in the freezing Ice Age, they have to hide from hunters and escape from a great cave bear. But when Jack and Annie are chased by a fierce sabretooth tiger and fall into a hunting trap, they're really stuck. Will anybody come to their rescue?