Bestselling author Jude Watson takes Amy and Dan on a thrill-packed ride for the 4th installment of the 39 Clues.

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A Clue found in Book 3 sends Amy and Dan jetting off to find out just what's behind the fierce rivalry between the Tomas and Ekaterina branches of the Cahill family. Was a Clue stolen from the Tomas branch? Where is it now? And most important, can Amy and Dan get their hands on it before their rivals do?

It's a wild race that will take Amy and Dan deep into the bowels of the earth . . . and right into the hands of the enemy.