The Cahill family has a secret. For five hundred years, they have guarded the 39 Clues ' thirty-nine ingredients in a serum that transforms whomever takes it into the most powerful person on earth. If the serum got into the wrong hands, the disaster would rock the world. So certain Cahills have always made it their mission to keep the serum safe, buried, locked away. Until now.

Thirteen-year-old Dan Cahill and his older sister, Amy, are the latest guardians of the Clues. They think they've done everything right, but a tiny mistake leads to catastrophe. The serum is missing and Dan and Amy have to get it back and stop who stole it . . . before it's game over. For everyone.