In the first 39 Clues book, Amy and Dan Cahill discovered that they're part of one of the most powerful families in human history. Passing up a pile of hush money, the siblings set out to uncover the 39 clues which would reveal the source of their family's awesome power. Here, in the sixth book, the siblings continue to uncover one clue at a time, encountering Cahill family branches and the mysterious Madrigals in the process. Amy and Dan cross the oceans on the trail of a famous aviator, but they find more than they're looking for. Their enemies are becoming more vicious, and the truths they discover more crushing than ever.