The early 1950s. Greta Thorburn and her husband Jim are at the beach with their children Barbara, Fergus and Tom. In the present day, an older Barbara explains to her lover Philip that her mother died 30 years ago. The young Barbara swims for the first time and Greta congratulates her.

1950s Edinburgh. Greta has been caught in a rainstorm. She meets her friend Wendy who takes her to the studio of artist Andrew Cunningham. Andrew is very pleased to see her and the two seem to have feelings for each other. Greta tells Andrew about her dreams and her poetry. Jim arrives and takes Greta home, where he confronts her about her occasionally erratic behaviour.

The present. Barbara and Philip discuss having children and argue over the nature of their relationship. In the 1950s, Greta returns to Orkney to nurse her father. He makes a quick recovery. Greta seems to enjoy being home, shares a drink with old friends and visits old haunts. Talk turns to Greta's mother.

Barbara explains that her grandmother, Mary Kelday, also died young. In 1930s Orkney, a young Greta accompanies Mary and her grandfather to the Mermaid pool. She is terrified when water starts to flood into the cave. The adult Greta tells Jim that a few years later, just before they met, Mary was swept off the same stretch of coastline and drowned. It seems both women were drawn to the sea.

In the present, Philip and Barbara visit the seaside. She explains that she is much more like her father than her mother. Later that night, she dreams of a white cat and flying through the streets of Edinburgh in her childhood dress.

Jim takes the younger Barbara to try on shoes. The older Barbara looks back on the beach holiday the family took just before her mother died. They are staying on Orkney, at an idyllic cottage by the sea. While her brothers play in the shallows, Barbara and Greta listen to a seashell. Jim did not accompany the family on their holiday, but he arrives later, in the middle of the night. Waking from a nightmare, Barbara has a premonition that something terrible has happened. Jim tells her that Greta has been found floating in the sea, still wearing her nightdress.

The adult Barbara wonders whether Greta could have been sleepwalking when she drowns. She explains to Philip that she and Jim have drifted apart since the latter remarried. They discuss the story of her mother and grandmother, and Barbara explains it is not a story yet because they are still a part of it. They decide to research Greta's published poetry.

Barbara photographs Andrew Cunningham for his retrospective at the Gallery of Modern Art. They reminisce about Greta. Andrew's retrospective proves to be a success. Barbara and Philip visit a club which prompts memories of the day when Greta's drowned. While Greta is being lifted out of the water, Jim and the children visit her room and find the unfinished poem she had just begun to write. Jim takes the poem and the family leaves.