When the Baker family arrives from Boston to Lake Fortune, in rural Maine, they are welcomed by a rickety cabin with an outhouse, a thunder and lightning storm, and a creature in the attic that masquerades as a ghost. But then true adventure begins when they discover a clue left in a Mason jar on a tiny island in the lake. Soon, they are on the path of an exciting treasure hunt, one that brings Charlie and his bossy big sister Clarrisa together for the first time. Through harrowing and exciting adventures, they realized they've just had the summer of their lives.