Dick and Dorothea Callum, colloquially known as "the D's", have come to stay at Dixon's Farm, on the lake shore above Wild Cat Island, for a week at the end of the winter holidays. Whilst there they see, and soon make contact with, the Swallows and the Amazons, who are themselves staying at their usual "native settlements" at Holly Howe and Beckfoot. The Swallows and Amazons explain that they have planned a winter expedition to the North Pole. Having built an Igloo in the woods above Holly Howe, they are waiting, with increasing desperation, for the "Arctic" to freeze before they have to go back to school. Impressed by Dick and Dorothea's skating ability, they invite the D's to join their expedition.