"The hunt for the 39 Clues has revealed many secrets about Amy and Dan's parents: Hope and Arthur's "vacations" were actually Clue-hunting trips; they were framed for murder in South Africa; and they were killed for one of their Clues. After Amy and Dan learned the truth about the Madrigals, they were relieved to know that, at the very least, their parents had been working for the good guys. However, there's a secret in their father's past that would shake them to the core. Arthur wasn't born into the Cahill Family. But, unlike most non-Cahills, he knew all about the Clues. And he had a secret mission of his very own..."