Unable to return to their normal lives despite the end of the Clue hunt, new Madrigal leaders Amy and Dan anxiously prepare for a retaliatory strike by the Vespers, who kidnap a number of Cahill family members and demand that Amy and Dan steal a famous painting.
Amy and Dan Cahill's quest to find the million dollars takes them to Vienna, where they must outwit their power-hungry relatives as they seek to solve a clue involving Mozart.
An escalating race to find the 39 Clues finds Amy and Dan pursuing a Clue guarded by thousands of the world's best-trained soldiers, an effort that separates them in explosively dangerous ways.