The thrilling conclusion to 39 Clues Part VI: Doublecross. A Cahill family member from the past is bent on revenge CHECKMATEThirteen-year-old Dan Cahill and his older sister, Amy, are running out of time. An exiled Cahill known as the Outcast has already re-created three of history's worst disasters, and he's saved the worst for last. If Dan and Amy can't find and stop the Outcast fast, he will initiate a full-scale nuclear meltdown.But as...