Miss Prissie Carmody has a full house. She was living quietly and happily with her ward, nineteen-year old Connie, in her South-west London flat when Prissie's second cousin appeared, his young wife in tow. Edris Tidson spent the war years as a banana grower on the Canary Islands, but now he and wife Crete are staying--perhaps permanently--with their frustrated but polite aunt. Connie, however, is quick to show her dislike of the invasion. One day, Edris reads in the newspaper a letter proclaiming the sighting of a naiad, or water-nymph, in the River Itchen. Excited by this whimsical account, Edris organizes a family excursion to Winchester (which Miss Carmody is to sponsor) in search of the nymph, even though no one shares his enthusiasm on this subject.