"'My cousin Kate from Budapest is coming to live with us. She's delicate and has had the measles!' shouted Jancsi to a friend as he and his father were driving to the station to meet her.

Jancsi had imagined Kate as a sort of fragile, fair-haired princess, but the little girl who stepped off the train had plain black hair, a smudgy face, and skinny legs. From the moment Kate arrived, things happened. She was afraid of nothing and full of ideas. When Kate looked most angelic, you could be sure she was thinking up some mischief."

Young Kate isn't at all what Jancsi and his family are expecting. She turns out to be an out-of-control little girl, sent by her father to live with her Uncle's family in the country. Kate's Uncle Marton is the "Good Master", a kind and respected man in the community. Her father has spoiled Kate since her mother died, and now he hopes his brother will be able to do something with her.

At first Jancsi is repelled by her unpredictable and disrespectful behavior. But he and Kate share many adventures on his father's ranch in Hungary—riding run-away horses, going to a Country Fair, celebrating Easter and Christmas in traditional ways. Eventually he learns to appreciate her spirit, and Kate learns to love and respect the people she has met. When her father arrives at the end of the book, he hardly recognizes his polite, self-controlled daughter, and she persuades him to move to the country to teach.