Amy and Dan Cahill's grandmother told them lots of things before she died—the average rainfall in the Amazon Basin, for example. But then there were a few things she "forgot" to tell them—like the fact that she was planning to launch a round-the-world hunt for 39 Clues leading to the source of great power. Now Amy and Dan are searching for the Clues, and the competition thinks they have insider information. Only a few people know the full truth-- Amy and Dan know nothing at all. In fact, they're the underdogs.