Elestren Nancarrow and her older sister Lowenna come from a hidden merfolk clan. Abused by their uncle in childhood, they were driven out and forced to hide their fish tails, living as humans. As adults, Lowenna jumps from the Brooklyn Bridge, and Elestren transforms her silver-blue tail to rescue her, but cannot afford treatment, leaving her sister with severe aftereffects.
To earn medical expenses, Elestren joins the Mordaunt family's high-paying maternity nurse selection. Using her merfolk hearing, she detects a foreign object in Ivo's airway, saves the child, and wins a two-month job worth four hundred thousand dollars. She then cares for Cressida, who suffered a mental breakdown after her husband Valerian's betrayal, and proposes a bet: she will make Valerian come back crying and begging her.
While protecting Ivo and helping Cressida regain confidence, Elestren hides her fish tail, scales, and abilities. The second young master, Auberon, notices abnormalities in her hearing, body temperature, and reaction to water, and begins investigating her. Meanwhile, clues to the "Little Snow Calamity" that the eldest young master, Isambard, has searched for years begin to intersect with this mermaid maternity nurse who has suddenly entered the wealthy family.
