"Thirty-Nine Clues" is a multi-media adventure series that combines books, cards, and an online game. The story follows the Cahill family, one of the most powerful families in history, as they compete in a global treasure hunt. The hunt is triggered by the death of the family matriarch, Grace Cahill, who reveals that the family’s secrets are hidden in clues scattered around the world. Each book in the series features different characters and locations, with the main protagonists being siblings Amy and Dan Cahill. As they solve puzzles and uncover clues, they learn about their family's history and the significance of the Cahill legacy. The series encourages readers to engage with the story through interactive elements, making it a unique reading experience.