Peggy Arnold, and younger twin siblings Mike and Nora, are living with a harsh aunt and uncle after their parents are thought to have been killed in a plane crash. In this first adventure, aided by an orphan boy, Jack, they have befriended, they run away to an island on a nearby lake, and together they make a new home constructed with the branches of a willow tree, this much-loved "living house" being one of the highlights of the story.