Dan Cahill and his sister, Amy, have learned a lot on the hunt for the 39 Clues, from the best way to escape a shark attack to how to break into an ancient Egyptian tomb. However, there's one secret they still need to uncover: Who are the Madrigals and what is their REAL goal? The siblings think they have everything under control, but they have no idea how powerful the Madrigals are...and how far this mysterious group will go to get what they want from Amy and Dan.