It's the middle of the night when Jeff spots someone dumping a large bundle into the lake. It's too dark to identify anyone -- or anything. But the cousins immediately suspect foul play, and plunge right into the mystery. Before they know it, the kids of Cabin Creek are in too deep. Everyone is a suspect -- and the cousins are all in danger.

Lost Island was off limits -- until now. Jeff and David are going to the desert island to search for clues. And hidden treasure! Town legend tells of a robber and a secret cave, but the brothers have to piece the truth together. With the help of their cousin, Claire, they'll get to the bottom of the mystery, no matter what they have to dig up.