The 39 Clues: Unstoppable Series is the third story arc in the bestselling young adult franchise The 39 Clues. It expands the story of Amy and Dan Cahill after the first two series The Clue Hunt and Cahills vs. Vespers. The series features 4 adventure books written by bestseller authors including Jude Watson and Gordon Korman.
First released in 2014, Countdown is an adventurous young adult novel written by Natalie Standiford. It is the 3rd and penultimate book in The 39 Clues: Unstoppable series, the third story arc of The 39 Clues Franchise.