Inspired by actual historical events, Alias Grace follows the story of convicted murderess Grace Marks. Born in Ireland, Grace immigrated to Canada at age twelve, along with her parents and siblings. Grace’s mother died on the journey and was buried at sea, a deep loss that Grace is still grieving in adulthood, and Grace’s father was an abusive alcoholic. Shortly after immigrating to Canada, Grace left her family to find work as a servant. While working at the house of Mrs. Alderman Parkinson, Grace befriended a fellow servant named Mary Whitney and was devastated when Mary died of complications from an abortion. After Mary’s death, a depressed Grace ultimately takes work at the home of Mr. Thomas Kinnear in Richmond Hill (today, a suburb of Toronto). Grace had a tense relationship with Nancy Montgomery, the housekeeper and Mr. Kinnear’s paramour, and with James McDermott, a hired hand at the household; however, she was on friendly terms with Jamie Walsh, a boy from a neighboring family, and Jeremiah the peddler, a traveling salesman. In 1843, when Grace was sixteen, Mr. Kinnear and Nancy were murdered under brutal yet mysterious circumstances. Grace and McDermott were both arrested and tried in the murder of Thomas Kinnear. McDermott was convicted and Grace was deemed guilty as an accessory to murder; since McDermott and Grace were convicted, the Montgomery murder was never tried. McDermott was hanged but Grace’s sentence was commuted to life in prison.
By the time the novel begins, in 1859, Grace has been in prison for over a decade, having also spent some time in an insane asylum. She claims to have no memory of the day the murders took place, though she remembers the days leading up to and after the murders. Though she remains imprisoned, Grace works at the Governor’s house several days a week, where she helps the Governor’s wife with sewing. The Governor’s wife is part of a group of people, led by a Reverend Verringer, who are working to obtain a federal pardon on Grace’s behalf. In an effort to strengthen his committee’s argument for a pardon, Reverend Verringer enlists the help of Dr. Simon Jordan, an American, Harvard-educated doctor with dreams of opening his own privately-funded mental asylum. Dr. Jordan begins interviewing Grace on a regular basis, hoping that allowing her to narrate her life story will help call up her lost memory of the murders. At the same time that he is conducting his interviews with Grace, Dr. Jordan guiltily begins an affair with his landlady, Mrs. Rachel Humphrey, whose husband, Major C. D. Humphrey, has abandoned her.
As Grace comes to the end of her story, Dr. Jordan begins to panic because he still cannot determine whether or not she is guilty. After speaking to the lawyer who represented Grace, Mr. Kenneth MacKenzie, and visiting Mr. Kinnear’s home in Richmond Hill, Dr. Jordan agrees to allow Dr. Jerome DuPont, a friend of Reverend Verringer’s (whom Grace recognizes as Jeremiah the peddler in disguise), hypnotize Grace. During the hypnosis, Grace begins speaking in Mary Whitney’s voice. Mary Whitney admits to possessing Grace and killing Nancy, accounting for Grace’s lack of memory about the murders and several “sleepwalking” spells that Grace had had since Mary died.
Dr. Jordan realizes that he is trapped. He cannot write a report detailing the hypnosis session because he will be seen as a quack and his dreams of opening his own asylum will be quashed. He also knows that Reverend Verringer will not let him off without writing a report on behalf of the Committee. Panicked about his situation, and distressed by his many interviews with Grace, Dr. Jordan persuades Rachel Humphrey that he is ill and needs her to fetch him a doctor. While she is gone, Dr. Jordan escapes back to the States on a train.
Dr. Jordan becomes tacitly engaged to a woman his mother picks out for him. Upon the outbreak of the Civil War, Dr. Jordan enlists as a military surgeon. He sustains a head injury and, according to his mother, loses his memory of his time in Canada. However, despite his claim to not remember treating Grace, Mrs. Jordan admits that her son mistakenly refers to his betrothed as Grace.
After twenty-eight years of imprisonment, Grace receives a pardon in 1872. She is transported to Ithaca, New York, where she marries Jamie Walsh, now a widower. At the close of the novel, Grace is on the verge of her forty-sixth birthday. She suspects she is pregnant—though she admits it might also be menopause or a tumor, like the one that killed her mother, saying, “It is strange to know you carry within yourself either a life or a death, but not to know which one.”