One day in the park, the Aldens' dog, Watch, comes across a mysterious box with a note inside. "Congratulations!" the note says. "You've found it!" The box is part of a fun new treasure-hunting game, and soon Henry, Jessie, Violet, and Benny are looking all over town for cleverly hidden boxes. But they're not the only ones on the trail for clues! When several of the treasure boxes go missing, the game is ruined for everyone. Who would steal the boxes—and why?