Iris Lockhart, a young unmarried woman with a successful vintage clothing store in 1990s Edinburgh, learns by phone that her great-aunt, Esme Lennox, will be released from a psychiatric hospital to Iris's care. The hospital, where Esme has been living for more than 61 years, is closing its doors; Esme has been evaluated as harmless.
Having never been aware of Esme's existence, Iris seeks information about her mysterious relative, and consults her grandmother, Kitty, who is hospitalized with Alzheimer's disease. Kitty has never once mentioned Esme, her younger sister, nor the conditions which have led to her institutionalization. When Iris confronts Kitty about Esme, her grandmother is vague and uncooperative. There is no one else to ask: Iris's father is dead; her mother has never heard of Esme.
The narrative shifts to colonial India, where sisters Kitty and Esme are young girls. Esme and her baby brother Hugo are left at home with servants while their parents and Kitty attend a house party. During their absence, the household is struck by typhoid, and both Hugo and the Indian nursemaid die, causing the rest of the servants to flee. Esme stays alone in the house by her baby brother's side for the entire three days. When her parents return, she hysterically refuses at first to let anyone take the baby from her.
After the burial, the family returns to Scotland. The girls are not allowed to speak of Hugo's death. Their lives are now consumed with the social obligations of their class in the 1930s — chiefly parties to introduce the girls to prospective husbands. The older Kitty is happy to comply with this arrangement; 16-year-old Esme is not. She begins to exhibit strange behavior, falling into trances. Intelligent and strong-willed, she often retreats into books, displaying no interest in the young men introduced to her.
One of Kitty's beaus, James, is intrigued by Esme's unconventional behavior, though Kitty is mortified by it. At a Christmas dance, James rapes Esme in a cloakroom after she spurns his advances. She is taken home, screaming uncontrollably. Neither Kitty nor her parents know that Esme has been raped, and she has no experience to explain the event. After the doctor interviews family members about Esme's overall behavior, she is taken away to Cauldstone, a mental hospital. The family, especially Kitty, is relieved to be rid of her, and they do not speak of her. Unknown to them, or even Esme, is the fact that she has become pregnant.
Shortly after Esme's removal to Cauldstone, Kitty marries Duncan. She wants to have a child, but they are unable to do so. Kitty decides to visit Esme secretly at the asylum, and discovers through the staff that Esme is pregnant. She elects not to see Esme, but to adopt her baby once born, and pass it off as ...