Never turn your back on the Cahills . . . The most powerful family history has ever known returns with a vengeance in The 39 Clues: Doublecross!The Cahills are the world's most powerful family, but their strength is being tested. A sinister man calling himself The Outcast has targeted the family and set them an impossible test. He's recreating four of history's worst disasters and challenging the young Cahills to find and stop the tragedies...