In this, the third book, Tolly and his grandmother are away (presumably in Cornwall). Two women have rented the house for the summer and ask three children to stay. Ida is the niece of one of the women; Ping and Oskar are refugees. The children are turned loose on the river, where they have many fine and imaginative adventures.
"Magic and mystery combine....Boston is an effortless stylist and her magic seems as simple as breathing." (The Washington Post)