Frank, Chase, and Griff were spending a late autumn afternoon overseeing a dozen Cub Scouts on a clean-up project when Chase came across an exciting discovery at the riverbank! Recent weeks with no rain lowered the level of the New Haven River, and this revealed something mysterious along the levee that had been hidden for more than fifty years!An antebellum mansion nearby became their focus as a strange box holding a cryptic message launched their search for its meaning. Add clues hidden inside an antique grandfather clock as well as discoveries from both sides of a sealed, rusty door, and the result is pure fun and mystery in a supremely satisfying adventure. The surprising outcome will keep you guessing until the final pages!Grab your flashlight, hop on your bicycle, and get ready for intrigue and twists in the fourth, exciting book of the Bon Air Boys Adventures Series!