WHEN CHARLIE BAKER'S PARENTS ANNOUNCE that they're taking him and his sister—the drama queen Clarissa—to a cabin in Maine for two weeks, Charlie thinks they're kidding. Then their mom adds something else. "There's no electricity," she tells them. "And you leave your cell phones at home." They weren't kidding.

When the Bakers arrive at Lake Fortune, they are welcomed by a rickety cabin with an outhouse, a thunderstorm, and a creature in the attic that masquerades as a ghost. "Is this a scientific experiment?" asks Clarissa. "Is the government paying us?"

But the true adventure begins when they discover a clue left in a mason jar on a tiny island in the lake. That clue—written by The Mystery Traveler—leads them to a second clue. Soon, they are on the path of an exciting treasure hunt, one that brings Charlie and the bossy Clarissa together for the first time. Not only do they bond as brother and big sister, they have the summer of their lives.