Rosanna is part of a close, extended Fenland family and the youngest of three cousins. She adores the impulsive, beautiful Merle, and Derry who dreams of sailing round the world. He cheerfully allows little Rosy to tag along one idyllic summer. For her, it is a time of puzzling illusions, like an escapade to see the floating moon mirrored in dark waters. Then one day Derry rows them in the boat to Ely Cathedral, which appears suspended above them in the shimmering heat. On their return, Rosanna is told she has a baby brother. This event, together with the outbreak of World War II, will change their way of life forever. As she grows up, the others turn to her when they are unhappy or in trouble. She has become Our Cousin Rosanna, who will pick up the pieces. To find her own happiness, she has to break free and finally make them aware that she has feelings too.