Iris Lockhart is a successful vintage clothing store owner in Scotland whose life is turned upside down when she receives a telephone call from a hospital for patients with mental illness asking her if she can come and collect a patient called Esme Lennox who is due for release; Iris has no idea who Esme is and is shocked to learn that they are actually related. Esme is her Great Aunt but nobody in the family has ever mentioned her. Even her father had never heard of his aunt. Uneasy, but reluctant to leave someone in need with nowhere safe to go, Iris collects Esme and takes her back to her own flat, against the wishes of her step brother Alex who is worried that Esme is going to be an axe murderer.
It turns out that Esme never really was insane despite incarceration in a mental asylum for over sixty years. As a child she was a free spirit who refused to conform to the strict parameters of Scottish society, unlike her sister Kitty who relished the prospect of marrying young and taking her place amongst the social elite. Esme not only wouldn't conform but was emotionally incapable of doing so. Academically gifted and a high flyer in school, she wanted to continue her education, not get married, and was held back by the time she lived in when women were the property of first their fathers and then their husbands. Two major traumas did not help her mental state; after being left at home for the weekend as a punishment whilst her parents and sister were out of town, she discovered her baby brother dead in his crib with their nurse dead beside him. She was holding the dead child for three days before she was discovered.
A couple of years later, she was at a New Years Party with the first boy she ever liked, James Dalziel, who rapes her. This causes a post traumatic stress reaction in Esme which her parents see as willfulness and to them, this is the same thing as madness. They have her committed to a hospital for the mentally ill. A month later it is discovered that she is pregnant. Esme delivers the child and wants to keep him but he is ripped from her arms minutes after the birth; she sees him leaving with her beloved sister, Kitty, whom she realizes betrayed her by making her sound mentally ill with the purpose of getting her out of the way whilst she got married. Kitty takes Esme's son as her own and raises him within her loveless marriage. She appears to do a good job as he, in turn, is a loving father to Iris, but he knows nothing of his history including the existence of Esme who is expunged from her family's history and abandoned at the hospital for the next sixty years. If the hospital had not been closing down for food she likely would have died there.
Iris and Esme like each other and form a bond. From her Grandmother, Esme's sister Kitty, who has dementia, Iris finally learns something resembling the truth, and realizes that Esme is not her great aunt at all, but her grandmother. After some days out by the seaside, Esme asks to visit with her estranged sister. Anxious to show that she trusts Esme and considers her a sane person who does not need supervision, Iris and her step-brother Alex leave them alone. The novel closes with paramedics and police arriving in Kitty's room because she has passed away; it is for the reader to interpret the circumstances under which this happened but as Esme is led away by police Iris is holding her hand as Esme has finally a family member who will not abandon her.