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?