In 1859, Grace Marks, a servant and convicted murderess in the deaths of her employer and housekeeper, is escorted from the Kingston Penitentiary to do housework for the prison's governor. Meanwhile, Simon Jordan, an ambitious New York doctor, arrives to meet a committee dedicated to pardoning Grace. Grace recounts to Simon her torturous childhood and emigration from Ireland. She claims to have no memory of the events surrounding the murders and describes her horrific prison conditions. As pressure mounts on Simon to deliver a favourable report, he becomes transfixed and bewildered by Grace's enchanting and persuasive nature.

In 1859, Simon dissects the events of the murders. In 1842, Grace begins her job as a servant for the Parkinsons. There she meets Mary Whitney, a passionate and determined young rebel. They are visited by Jeremiah, a charming peddler who reads Grace's palm and gives a dark yet optimistic prediction of her future. Mary confides in Grace that she has become pregnant (by George Parkinson, the son of the household, who refuses to marry her). When Mary dies from a back alley abortion, Grace enters a coma-like state from the trauma, waking days later and suffering from identity lapses, confusion, and hysteria.

In 1859, Simon spends more time with Grace trying to extract her repressed memories, but she remains unreadable. In 1842, after attending Mary's funeral, Grace meets an attractive and spirited young woman, Nancy Montgomery, who offers Grace a position at the Kinnear farmhouse. There, Grace is introduced to Kinnear's brooding stable hand James McDermott, along with a freckled young neighbour, Jamie Walsh. As Nancy becomes increasingly irritable and unpredictable, Grace suspects that she is also Kinnear's mistress.

In 1859, Simon becomes intimate with his landlady, Mrs. Humphrey. In 1843, Jamie Walsh confesses his love for Grace. Kinnear disapproves, mocking her after spying on the pair with his telescope. Grace is then visited by Jeremiah, who has decided to become a hypnotist, and offers her a life with him away from the farm.Grace ultimately declines, as Jeremiah does not wish to be married. As tensions rise on the farm between Grace and a now pregnant Nancy, McDermott reveals that he plans to kill both Nancy and Kinnear and throw their bodies into the cellar.

In 1859 Simon tries desperately to decipher Grace's guilt or innocence as she discusses the day of the murders with lapses in her memory. With time running out, Jerome DuPont (previously Jeremiah) convinces Grace to let him try neuro-hypnosis on her. In 1843, Grace reveals McDermott's plan to Nancy, who dismisses it as McDermott "bragging and boasting." Nancy and Kinnear are brutally murdered and Grace flees with McDermott to New York, where they are caught and arrested.

In 1859, the committee, along with Simon, Grace, and Jerome DuPont begin the anticipated neuro-hypnosis experiment. Once Grace is fully hypnotized and asleep, Mary Whitney's voice echoes from within her body, joyfully claiming to have possessed Grace on the day of the murders, and driving her to assist in the murders. Completely unravelled, Simon evades the committee and flees to Toronto, where he joins the Union Army only to return home, heavily shell-shocked and concussed. In 1872, Grace gets unexpected news and contemplates her future.