In the second book of this code-busting series, Marly, Sai, and Isla are back on the hunt. After finding Mr. Summerling's hidden treehouse, they've discovered a new set of clues for them to follow, leading them straight to the heart of their mysterious neighbor's life: his house. The same house Marly has been living next to all of these years, which, they discover, is filled with hidden rooms and staircases. With just a few scraps of papers as clues, and Mr. Summerling's angry son on their tails, can the Treasure Troop find the answers they're looking for?