When Karlsson comes flying through Lillebror's window they are up to all sorts of high jinks.

Karlsson is a very short, very portly and overconfident man who lives in a small house hidden behind a chimney on the roof of a very ordinary apartment building, on a very ordinary street in Stockholm. When Karlsson pushes a button on his stomach, it starts a clever little motor with a propeller on his back allowing him to fly. Karlsson is the best at everything, at least according to himself. But there is, in fact, one thing at which he excels: being a playmate to a young boy named Svante, but nicknamed Lillebror (little brother), who lives in the apartment building with his family. Karlsson is quite mischievous, often getting Lillebror in trouble, but everyone in the end - even Lillebror's family, his cranky uncle, and their dour housekeeper - grow to like Karlsson and appreciate his sense of humour, energy, and good nature.