A dinner invitation for Digger, who is accompanied by the very pale Uncle Benjamin, gets her involved with a recently discovered relative of their elderly friend Thelma, whose brother died during World War II. DNA testing has led his until-now-unknown great-grandson, Peter, to Maple Grove in the Western Maryland Mountains. Thelma enlists Digger's help as she gets to know Peter. But his presence is not the only surprise. Letters Thelma recently received from another brother's estate raise questions about what happened to some of her parents' prized possessions. Were they submerged with the family's old farm house, which became part of Deep Creek Lake when it was filled in the mid-1920s? Digger and Uncle Benjamin see no way to learn more, but night visits to the lake by an unscrupulous duo may mean someone else is looking for valuables to plunder. But how would they know it exists, and what would convince them to dive for it? And what will happen to Digger or her friend Marty if they get in the way of the treasure seekers?