The intricately narrated story involves a mermaid who comes ashore on the southern coast of England in 1899. Feigning a desire to become part of genteel society, the mermaid's real design is to seduce Chatteris, a man she saw "some years ago" in "the South Seas—near Tonga," who has taken her fancy. This she reveals in a conversation with the narrator's second cousin Melville, a friend of the family that adopts Miss Waters. As a supernatural being she is unimpressed with the fact that Chatteris is engaged to a Miss Glendower and is trying to make amends for his wastrel youth by entering politics. Chatteris is unable to resist the mermaid's alluring charms, though succumbing means his death.