In a prologue set in November 1950, an unnamed girl hurries along a deserted road in rural Vermont toward Idlewild Hall, a boarding school for unwanted girls. A figure in a long black skirt and veil, the school's resident ghost known as Mary Hand, closes in behind her with unnatural speed. The girl plunges into the woods, dropping her suitcase, and screams as she nears the school grounds.
The story shifts to 2014. Fiona Sheridan, a 37-year-old freelance journalist in Barrons, Vermont, sits in her car on the same road at three in the morning on the birthday of her sister Deb, who was murdered and left in the sports field at the abandoned Idlewild in 1994. Tim Christopher was convicted of the killing, but Fiona has never stopped questioning the investigation. Her boyfriend, Jamie Creel, a Barrons police officer whose father and grandfather both served as chief of police, tells her the property has been sold to Margaret Eden, a wealthy widow, and is being restored as a boarding school.
Fiona pitches a story on the restoration to the editor of the local magazine
Lively Vermont. Her father, Malcolm Sheridan, a renowned journalist now over 70, helps her arrange a tour of the property through Margaret's son, Anthony Eden, who manages the project.
The narrative alternates between Fiona's investigation and the lives of four roommates at Idlewild in 1950. The school houses 120 girls considered throwaways: illegitimate daughters, immigrants, and girls who misbehave. In room 3C, four 15-year-olds form a bond through shared trauma. Katie Winthrop, beautiful and defiant, was sent away after being sexually assaulted by a neighborhood boy. CeCe Frank, the illegitimate daughter of a wealthy banker named Brad Ellesmere, nearly drowned when her mother tried to kill her as a child. Roberta Greene stopped speaking for months after witnessing her Uncle Van, a traumatized war veteran, with a pistol in his mouth. Sonia Gallipeau, a quiet French immigrant, survived Ravensbrück, a Nazi concentration camp, as a child after her mother was executed there. The girls share encounters with Mary Hand, who reaches into their minds and conjures their deepest fears.
During Fiona's tour with Anthony, the construction crew unearths remains in the old well: a teenage girl with her skull smashed in and a name tag reading "Sonia Gallipeau." French records reveal Sonia's father died in Dachau in 1943 and she traveled alone to America in 1947, her previous residence listed as Ravensbrück. Fiona contacts Ginette Harrison, a British historian researching the camp, who theorizes that Sonia, with no family or advocate, was an ideal victim for someone who wanted to kill without consequences.
Sarah London, a retired teacher who spent 29 years at Idlewild, confirms she always believed Sonia was murdered rather than a runaway and provides the names of Sonia's roommates. When the school closed in 1979, Sarah rescued decades of student records. Fiona and Jamie retrieve the files, which include the headmistress's conclusion that Sonia ran away and a note that her suitcase disappeared from the headmistress's office. In New Hampshire, Roberta Greene, now a retired lawyer, confirms the girls always knew Sonia was killed and describes Mary Hand's power to show people their deepest fears.
Fiona's investigation also reopens Deb's case. Stephen Heyer, a man struggling with addiction, reveals that his sister Helen was beaten nearly to death by Tim Christopher in 1993 and has been in a care facility ever since, unable to speak. The police questioned Tim once and dropped the case. When Fiona brings this to Jamie, they fight bitterly. Jamie later admits he pulled Helen's file and found the investigation incomplete, with his father Garrett Creel's name on it. He confesses to a pattern of his father's corruptions and says he was looking for a way out of the department's toxic culture when he met Fiona. They reconcile.
In the Idlewild records, Fiona discovers that before the school existed, the site held a farmhouse belonging to the Hand family. In 1914, 16-year-old Mary became pregnant and gave birth to a stillborn baby, possibly killed by her parents. Mary was banished into the cold and found dead the next morning in the ruins of an old church. She and her baby were buried on the spot that became the school's garden.
Fiona also finds that a woman named Rose Albert was tried in Burlington in 1973 for being Rosa Berlitz, a guard at Ravensbrück. Rose worked in a travel agency blocks from where Sonia's relatives lived. When Sonia left her relatives early and went to change her bus ticket, Rose likely recognized a former prisoner who could expose her identity. The trial ended in acquittal, and Rose died of a heart attack that year.
In the 1950 timeline, CeCe has a terrifying encounter with Mary Hand and reaches a crucial realization: Sonia saw her killer's real face behind Mary's veil on Old Barrons Road, ran, and was caught. The three surviving girls steal Sonia's suitcase from the headmistress's office and find her notebook of drawings inside, proof she would never have left it willingly.
Fiona visits Lionel Charters, who lives across from Idlewild. Lionel reveals that on the night of Deb's murder, Garrett Creel distracted witnesses at the old drive-in while Tim Christopher sat in his cruiser. Garrett helped Tim dump Deb's body and intimidated witnesses into silence. Before Fiona can leave, Garrett arrives, forces her into his car, and confesses to covering up Tim's crimes because the Christophers were powerful allies. He tells Fiona he intends to kill her.
Fiona escapes the moving car. Garrett catches her and tries to strangle her, but she fights free and sprints to the Idlewild property. Inside, the ghost of Sonia silently leads her to room 3C, where she collapses. Jamie and backup officers burst in after Malcolm, alerted by Lionel, called Jamie and directed the team to Idlewild. Garrett is arrested after firing at the officers.
Recovering in the hospital, Fiona confirms her growing suspicion: Margaret Eden is Katie Winthrop. Katie visits with Roberta and CeCe and reveals the full story. She married CeCe's half-brother Joseph Eden, heir to the Ellesmere fortune, and used his wealth to send Roberta to law school, CeCe to college, and to fund decades of investigation into Sonia's murder. In 1973, CeCe recognized Rose Albert's face from a portrait Sonia had drawn in her notebook of Ravensbrück guards. The three women confronted Rose, who admitted to killing Sonia before dying of a heart attack during the confrontation. Katie bought Idlewild not to restore it but to find Sonia's body; now she intends to demolish the property.
Fiona and Malcolm launch a campaign exposing Garrett's corruption, prompting a state investigation. Jamie cooperates against his own father and accepts his policing career is over. Sonia receives a proper burial, her three surviving friends standing vigil. On a cold December day, a crew digs up the school garden and unearths a coffin containing the remains of Mary Hand and her baby. Fiona sees Mary standing at the edge of the trees, holding a swaddled infant. When the coffin is opened, Mary vanishes. The novel closes with the windblown field, the blank winter sky, and silence.