Set on a small, fictional Welsh island in the years before the Second World War, the novel opens with a lyrical prologue describing the rhythms of island life: migratory seabirds in spring, houses repainted white in summer, sudden cold in autumn, and winter confinement by the hearth. This cycle frames the story of Manod, a young woman caught between the shrinking world of her island home and a longing for something beyond it.
One September morning, a whale becomes stranded in the shallows. By dawn it lies on the beach, drawing crowds. Some fishermen say it went off course; older islanders debate whether it is a good or bad omen. The whale's body lingers for months, slowly decaying as the narrative unfolds.
Manod was born on January 20, 1920, on an island three miles long and one mile wide, home to 12 families, a Baptist minister named Reverend Jones, and a Polish lighthouse keeper named Lukasz. She lives at Rose Cottage with her father, Tad, a lobster fisherman who habitually calls her by the dog's name, and her younger sister, Llinos, an unusual child who collects animal bones and assembles them into skeletons. Their mother is dead. Manod wears her mother's old dresses and copies patterns from mainland magazines, aware that islanders dress a decade behind the times. No one on the island knows how to swim. Over supper, Tad pressures Manod to consider marriage; she resists, asking who would look after Llinos.
Her closest school friend, Rosslyn, has already left to marry on the mainland, sending one letter asking what Manod planned to do; Manod never replied. Manod was a strong student whom Sister Mary, one of the teaching nuns, called bright and lent English novels. She has been sleeping with Llew, the only other person her age on the island, but does not love him. He asks her to marry him repeatedly; she neither agrees nor refuses. When Llew leaves for mainland work, Manod feigns illness and stays in bed. She does not hear from him again.
Shortly after, a white boat arrives carrying Edward and Joan, English researchers from Oxford who have arranged through Reverend Jones to write a book about the island's customs, songs, and stories. They need a translator. Joan seeks Manod out on the cockle-gathering flats, describes the island as wonderful and otherworldly, tells Manod about train travel and city life, and calls her English perfect. Manod asks if women can attend university; Joan says of course. Manod accepts the position.
Edward presents a research outline covering food, customs, songs, tales, and geography. Manod gathers fishermen to sing traditional songs in the chapel, translates lyrics and folktales, and records her own family history. Interspersed throughout the narrative are transcriptions from the researchers' disc recordings: folktales about bones that turn the world upside down, a seal pup raised as a drowned woman's son, and a selkie, a seal-woman of Celtic folklore, who escapes when her children find her sealskin.
Edward invites Manod to sing most mornings before Joan arrives. Over coffee, they discuss the threat of war and his childhood in a vicarage. A quiet intimacy builds between them. Joan, meanwhile, gives Manod gifts, including a lipstick in a royal blue casing. When Manod puts it on Llinos, she has to look away: Llinos looks just like their dead mother.
The novel fills in that mother's story in fragments. After Llinos's birth, Mam grew thin and withdrawn. A mainland doctor produced only pink pills; Mam then announced the Angel Gabriel had opened her back with a sword and released wings, declaring herself saved. She was eventually found dead on a beach 20 miles away. When Manod and Tad viewed her body in a morgue, her skin was silver, her legs fused into one stump.
Joan's romanticized view of island life creates growing friction. At dinner, she rhapsodizes about islanders living in harmony with nature and mentions contacts at the National Salvage Council, an organization collecting whale products for wartime use. Edward privately tells Manod that Joan is a follower of the British fascist Oswald Mosley who attended rallies and holds dangerous ideas.
In November, Manod begins meeting Edward at night in the chapel outbuilding. He describes his mainland life; she tells him she could come with him and Joan, and he says she could do anything. She also discovers the researchers' dishonesty: staged photographs labeled as authentic, strangers captioned as a family picnic. The exploitation intensifies when Joan and Edward direct a fisherman to pose in dangerously rough water, pretending to catch fish bare-handed. Manod protests. Joan responds by noting Manod's lipstick on Edward's clothes and accusing her of chasing men. Manod retaliates by revealing that Edward called Joan a Fascist, then tells Joan the island she imagines does not exist. Joan dismisses her.
A letter arrives from Llew, who has enlisted in the army after failing to find steady work. Tad's catches dwindle. In December, men from the National Salvage Council hack the whale apart and carry away its blubber and organs, leaving only the bones. A ship of children bound for Ireland stops overnight at the island; Llinos later finds a paper label on the beach for a child named Ruth Stern, age three.
Edward appears at Manod's window, revealing he and Joan are leaving in two days. Manod says she thought she was coming with them. Edward admits he may have led her to believe something he did not intend and offers the consolation that her photograph will appear in the book. Manod asks for embroideries she contributed to the project, but Joan never returns them. On departure day, Manod finds the researchers already gone. In their empty room she discovers field notes describing her father by his height, Manod as likely to be married off, and Llinos's teeth as chipped to points. Joan's notes romanticize islanders as possessing an innate wisdom. Manod does not recognize the people they describe.
Manod helps Llinos through her first menstrual period in the dark, a quiet maternal act. When Llinos asks her to bring something from England, Manod lies and promises she will, unable yet to admit she is not leaving. For the Mari Lwyd, a Welsh midwinter wassailing tradition in which decorated horse skulls are carried on poles from house to house, Manod cooks lobster and helps Llinos into a black dress, plaiting her hair with green ribbon. Three horse skulls are paraded in candlelight, and behind them, carried on six men's shoulders, is the whale's skull. Manod whispers something her mother always said into the whale's ear. The skull nods twice and shakes its head; the crowd cheers. For the first time, Manod remembers her mother's face clearly.
Unable to sleep, Manod sews through the night. At dawn she climbs the cliffs and watches the sun rise, something dark breaching the water's surface. She resolves to write back to Rosslyn at last: She will go to the mainland, take Llinos if she will come, find work, buy a house, and never marry. She closes her eyes, and the sea behind her eyelids turns brilliant red. When she opens them, the body is gone. The water bathes in its own light.