The Mysteries Inside Grandpa's Tackle Box

Tomboy Cathy Moore and her brother Tyler are best friends. Until the summer when she turned nine. Their dad gave Tyler their grandpa's old tackle box, and Tyler suddenly transformed into a swashbuckling pirate collecting strange treasures and sneaking out after midnight to hide his loot. Leaving Cathy behind, wondering where, on the hill beside their house, he buries his box.
Cathy, Angel Beach's self appointed Harriet the Spy with Tyler as her assistant, starts checking into a few strange things happening around town. A couple of them, Tyler's reactions appear more stand-offish, than helpful. Cathy begins to wonder if Tyler's involved. But when Tyler is pushed to his death, she promises his memory that she will find out who is responsible even through threats to her own life.