When Old Grace Cahill kicks the bucket, her will reveals the first in a series of secrets that will take her heirs on an adventure around the world. Key among them are Amy and Dan Cahill, a brother and sister team determined to discover Grace's secrets. With the help of their glitter eye shadow-wearing, nose-ring-rocking au pair, Nellie, the three of them travel to Philadelphia and Paris on the trail of that guy on the front of the $100 bill: Benjamin Franklin.
Along the way, they have to evade their conniving relatives. The Starling triplets interfere in Philadelphia, but are knocked out in an explosion meant for Amy and Dan. In Paris, former KGB agent Irina Spasky leads them into a trap; the Holts, a family of meatheads, almost get Amy and Dan buried alive; and their Uncle Alistair—alleged inventor of the microwave burrito—repeatedly betrays their trust. All along the way they're watched from a distance by the super creepy man in black who is secretly in cahoots with William McIntyre, Grace's attorney.
Eventually, Amy, Dan, and Nellie find themselves in the Maze of Bones, an intricate series of Catacombs under Paris. There, they discover the origins of the four branches of the Cahill clan and a clue that will lead them to yet another country in pursuit of secrets behind another historical figure: Wolfgang Amadeus Mozart in Vienna, Austria. The hunt for clues is to be continued…