The novel alternates between two timelines set in Ontario, Canada, connected by a women's prison and the fate of an inmate named Annie Little. In 1961, a young journalist goes undercover at the Mercer Women's Prison in Toronto to expose abuse. In 1996, a detective in rural Huron County investigates an unidentified skeleton unearthed in a small-town cemetery. A 1987 prologue foreshadows the mystery: An unnamed woman watches a man kneel at night in the Millgate Cemetery at the base of a large tree, pressing his hand to the grass in grief before disappearing.
In May 1961, Emily Radcliffe is a 24-year-old editorial assistant at
Chatelaine, a Canadian women's magazine that covertly publishes feminist articles beneath domestic-themed covers. When a delivery driver brings a crumpled note from an anonymous inmate at the Mercer Women's Prison alleging starvation, medical experimentation, and unjust imprisonment, Emily and her editor-in-chief, Doris Anderson, investigate. They discover the note references the Female Refuges Act (FRA), an Ontario statute that allows a parent to have a daughter under 25 declared unmanageable, with sentences of up to two years. Emily visits the prison and meets June Jones, a madam being released, who confirms the conditions are terrible and suggests Emily get herself committed if she wants the truth. Emily resolves to go undercover.
Doris reluctantly approves. Emily's father, William, a war correspondent, agrees to bring her before a judge under the FRA, and her mother, Bess, eventually relents. Emily breaks off her relationship with her boyfriend, Jem. At the courthouse, the judge barely acknowledges Emily and sentences her to six months, twice what she anticipated.
The 1996 timeline introduces Detective Rachel Mackenzie of the Ontario Provincial Police (OPP). A gravedigger at the Millgate Cemetery accidentally unearths a decades-old skeleton near a large maple tree. Rachel's supervisor assigns her to the case to push her off another investigation, and she begins tracing the body's origins with her rookie partner, Stevens.
Rachel's history emerges through flashbacks. She grew up with her grandmother Dora in Bayfield after her mother, Mary, was deemed unfit by Children's Aid. Mary drifts in and out of Rachel's life, exhibiting untreated mental illness, and Rachel endures traumatic episodes, including witnessing a miscarriage during which Mary accuses Dora of poisoning her. In the summer of 1987, Dora reveals to Mary's fiancé, Kevin, that Mary was responsible for the death of her younger brother, Walter Jr., who fell from the property's cliff while Mary was babysitting him. The argument turns violent: Mary attacks Dora, kicks Rachel when she intervenes, and shoves Dora off the cliff to her death. Mary is convicted of second-degree murder. Rachel spirals into alcoholism and depression until Detective Tom Stevens, her current partner's uncle, guides her toward rehab.
Inside the Mercer, Emily endures a painful pelvic examination by Dr. Eris Stone, the prison physician. She befriends Annie Little, a psychiatric inmate imprisoned for 15 years following a bout of postpartum psychosis, a temporary condition after childbirth. Annie recovered within months, but the law allows the physician to deny release indefinitely, and Stone has done exactly that. Annie's husband divorced her, and she has never known her son, Gregory, beyond infancy. Emily also befriends inmates whose sentences reveal the FRA's cruelty: Eliza, a teenager who prefers the prison to her abusive home; Gertrude, imprisoned for being a lesbian; and Lizzie, a mother given the choice between sterilization and prison. Emily secretly records observations on toilet paper and hides them inside the water tap in her cell.
Rachel traces a serial number on the casket to the Mercer Women's Prison through a coffin maker's records. At the Ontario Archives, she reviews prisoner registers and isolates five women who died or were unaccounted for at the Mercer between the late 1950s and 1962.
Emily discovers that Stone is running an unauthorized drug trial, deliberately infecting inmates with venereal disease during exams and testing an experimental treatment. June Jones returns to the prison and, after Stone doubles her bribe price, decides to help Emily expose the doctor.
With days left in her sentence, Emily needs documentary evidence. June orchestrates a fire as a diversion while Eliza picks the locks on Stone's office. Emily retrieves patient files revealing that Annie's mother, Helen Sharrock, submitted four appeals over 10 years for Annie's release, each suppressed by Stone. When Emily tells Annie, Annie is overwhelmed to learn her mother never abandoned her. Emily confronts Stone, revealing she is a journalist and demanding Annie's release. That evening, Annie tells Emily that Stone gave her an unfamiliar medication. Annie loses consciousness and dies in Emily's arms in the dining hall. Emily accuses Stone of murder, but the matrons restrain and sedate her. She wakes in an isolation cell, reclassified as a psychiatric inmate under the same provision Stone used to hold Annie.
On Christmas Eve, June orchestrates a full prison riot after revealing Emily's identity and Stone's crimes to the inmates. Eliza picks the lock on Emily's isolation cell, and Rose, an inmate Emily previously feared, helps free her. Emily dons Stone's white lab coat, retrieves her hidden notes, and poses as a doctor to secure a police escort to a hospital, from which she and June escape by taxi.
Emily arrives home on Christmas Eve and writes the article over 10 days. Her mother reveals that Emily's twin sister, Eleanor, after giving birth, spoke of not wanting to live and was too frightened to seek help for fear of institutionalization, a disclosure that deepens Emily's resolve. The article, titled "Murder at the Mercer," runs in the February 1962 edition of
Chatelaine and triggers a grand jury investigation leading to the Mercer's closure. Stone is stripped of her medical license and imprisoned, but her responsibility for Annie's death is never legally established; the official ruling remains suicide.
In 1996, Rachel eliminates three names through coroner's records and traces Emily Radcliffe to
Chatelaine. Doris Anderson confirms Emily went undercover and that Annie died in her arms. Rachel and Stevens visit the retired Emily at her Rosedale home, where Emily confirms Annie's identity and describes lasting psychological effects, including chronic anxiety and symptoms she now recognizes as traumatic stress. Rachel also visits Mary in prison for the first time in 10 years; Mary refuses to confirm or deny whether she deliberately killed Walter Jr., insisting that Rachel prefers a clear picture to the gray truth.
Reverend Holland, the former minister at Millgate Methodist Church, reveals that after Annie died and her ex-husband refused to claim the body, Annie's mother wanted her brought home. Fearing the congregation would reject a burial for a woman whose death was classified as suicide, Holland helped Helen inter Annie secretly near the family plot without a headstone. Rachel and Stevens decline to pursue charges. Through the former Sharrock property, they locate Gregory Little, Annie's son, and bring Emily to meet him. Emily tells Gregory about his mother's warmth, her love of strawberries, and how her death galvanized the other inmates. Gregory's daughter is present and bears the name Annie.
Annie's remains are cremated and buried with her parents at the Millgate Cemetery, her name added to the family headstone. Gregory's family wears purple, Annie's favorite color. Emily attends with her husband, Howard, and places fresh strawberries at the base of the stone. Gregory asks if she thinks Annie is looking down on them. Emily looks at his daughter, sees Annie alive in her family, and tells him she hopes so with all her heart.