Grace Marks, the convicted murderess, has been hired out from prison to serve as a domestic servant in the home of the Governor of the penitentiary. A Committee of gentlemen and ladies from the Methodist church, led by the minister, hopes to have her pardoned and released. Grace cannot remember what happened on the day of the murders, and she exhibits symptoms of hysteria, so the minister hires Dr. Simon Jordan, a psychiatrist, to interview her, hoping he will find her to be a hysteric, and not a criminal. An arrangement is made so that Jordan will interview Grace during afternoons in the sewing room in the governor's mansion.
Jordan tries to lead Grace into talking about her dreams and her memories, but she evades his suggestions, so he asks her to start at the beginning, and she proceeds to tell him the story of her life. Grace tells of early childhood in Ireland where her father was often drunk and her mother often pregnant and Grace had to take care of the younger children. She tells the doctor details of the filthy crowded conditions in the hold of the ship where her mother sickened and died. In Canada, because her father continued to spend his earnings on alcohol, she and the children nearly starved and with her mother gone, Grace's father began abusing her and even at one point attempted to rape her. Jordan listens but is impatient, viewing her early privations and abuse as irrelevant to the case.
As a serving girl, Grace tells the doctor how she met Mary Whitney, then her roommate and only friend. Mary taught Grace how to act the role of a servant, and joked with her about the family's upper class airs, when nobody else was listening. Giving motherly advice on how to stay out of trouble with young men, Mary told her "if there is a ring, there had better be a parson" (p. 165). Mary herself became pregnant – presumably by a son of the family – and died from a botched abortion. Grace had helped Mary get home and into bed, but awoke the next morning to find Mary dead. Grace was troubled afterwards by the idea that she should have opened the window during the night when Mary died to let her soul out (p. 178).
Grace continues her story in vivid detail, making an effort to keep the doctor interested. He is aroused by Grace's descriptions of James McDermott's advances and Thomas Kinnear's affair with Nancy Montgomery. The doctor's landlady, whose drunken husband had by then left her, throws herself at him. He is not attracted to her and turns her away several times. He gives her money so she can keep the house thinking she will stop bothering him, but that only provokes her to try harder until she finally seduces him once, only for him to tell her that he wished it to be with someone else and never with her.
A spiritualist on the committee has long since proposed that a Dr. DuPont, "Neuro-Hypnotist", should put Grace in a trance and arouse her unconscious memory. Jordan, now mostly concerned with escaping the designs of his landlady, can no longer dissuade the committee. It appears to all present that after DuPont puts Grace to sleep, the voice of Mary Whitney takes over, gleefully telling everyone she haunted Grace because her soul was not freed when she died. She said she possessed Grace's body on the day of the murders, and drove McDermott to help her kill Montgomery and Kinnear. She says Grace does not remember because she did not know what happened. Jordan allows that there have been some scientific reports of a "double personality" phenomenon, but he evades the committee's request for his report and leaves town, claiming his mother is ill. He promises to send them the report, but returning home, he promptly joins the Union Army. After he is wounded in the war, he moves into his mother's house. The last word he says is "Grace". Grace eventually is pardoned, and according to the novel she changes her name and begins a new life in the United States.
