Lillebror moves with Twigson and his family to the western part of Norway, but he dreams of even bigger expeditions, like going to the North Pole.

This time, they travel on holiday with Aunt Thea to Summer Island. There they meet Eddy who is an expert on all the challenges they can face on a magical island. It is rumored that a sea worm has been observed in the fjord, and it will not be long before the energetic sea worm expert Blegen shows up, with his strange devices and instruments. It will be an exciting summer holiday with friendship put to the test, exploration in nature and the hunt for the mysterious sea worm.

When Lillebror's mother is mysteriously hurt in a biking accident, Lillebror and Knerten turn the town upside down trying to find the culprit.

Twigson follows the story of a young boy who discovers an imaginary friend in the form of a salesman. Together, they embark on a snow-filled adventure to save money, renovate a house, and navigate family relationships. Set in the 1960s, the film is based on a novel and also explores the themes of friendship and buying groceries.

I den tredje filmen om Knerten har Lillebror og familien hans flyttet. Knerten har fått en sønn, Lille-Knerten, og lykken er stor. Men mor og far er bekymret for Lillebror. De har bodd i Bessby i et par måneder, men han har fremdeles ikke fått seg noen nye venner. Lillebror bryr seg ikke. Han liker ikke de andre guttene i gata uansett, og dessuten har han jo Knerten. De to gjør en avtale om ikke å ha noen andre venner. For alltid. Men en dag forsvinner Knerten sporløst…