As summer begins to wane, the Bartletts leave the lakeside community of Wolfhollow, New Hampshire to take a last minute vacation to the seashore. A half mile from the beach sits Mystery Island. This fog-enshrouded island happens to be the destination on a pirate treasure map the kids recently discovered. However, to find the buried treasure, Fourteen-year-old Leah Bartlett and her two siblings, twelve-year-old Avery, and nine-year-old Grayson must decipher a secret code. Along the way they meet new friends, encounter rough seas, and uncover a treasure that is different from anything they could possibly imagine.
The local newspaper nicknamed the Bartlett children the 'Rucksack Kids' for the canvas backpacks they each carry. The rucksack contents, like the Bartlett children's different personalities, contain a variety of tools and gadgets they use to help solve mysteries.