The in Too Deep (the 39 Clues,

Fourteen-year-old Amy Cahill and her younger brother, Dan, head to the Land Down Under to discover what their own mother and father knew about the hunt for the 39 Clues. But following in their parents' footsteps brings up lost memories for Amy so awful that she can't share them . . . even with Dan. Haunted by the ghosts of their past, chased by deadly competitors, Dan and Amy can't see who is an enemy and who is a friend. Their blindness leads to a terrible mistake . . and the death of a hidden ally.