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. "But we're city kids," Clarrisa protests. "I'm allergic to trees!" Then Mrs. Baker adds something else. "There's no electricity," she tells them. "And you leave your cell phones and ipads home." They weren't kidding. "It's on a lake," says his father, as if having water would make up for it.

When the Bakers arrive at 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. "Is this a scientific experiment?" asks Clarissa. "Is the government paying us?" But then 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 an old apple tree, and a second clue. Soon, they are on the path of an exciting treasure hunt, one that brings Charlie and the bossy Clarrisa together for the first time. Not only do they bond as brother and big sister--especially after a harrowing experience in which Charlie rescues Clarissa--they have the summer of their lives. When The Mystery Traveler's identity is finally discovered, no one is more shocked than Charlie. They are city kids who can now find the Big Dipper and Jupiter in the sky, who know that strange sound is made by loons, and what "fetch" a pail of water means.

They wish the two weeks were two months. But they also love the city. They return to Boston knowing that the cabin and more adventures will be there for every vacation.