Armadale first appeared as a serialisation in the Cornhill Magazine, issued in twenty monthly instalments from November 1864 to June 1866. It was serialised almost concurrently in the United States, appearing in Harper's New Monthly Magazine between December 1864 and July 1866. It first appeared in book form in May 1866 (Smith, Elder & Co., two volumes).
Summary
In the German spa town of Wildbad, the 'Scotchman' Mr. Neal is asked to transcribe the deathbed confession of Allan Armadale; his story concerns his murder of the man he had disinherited (also called Allan Armadale), who had subsequently married the woman he was betrothed to under false pretences. Under Armadale's instructions, the confession is left to be opened by his son once he comes of age.
Nineteen years later, the son of the murdered man, also named Allan Armadale, rescues a man of his own age, Ozias Midwinter. The stranger reveals himself to the Reverend Decimus Brock (a friend of Allan through his late mother) to be another Allan Armadale, the son of the man who committed the murder. Midwinter tells the Rev. Brock of his desperate upbringing, having run away from his mother and stepfather, Mr. Neal. The Rev. Brock promises not to disclose the relationship between the young men, who become close companions. Midwinter remains haunted by a fear that he will harm Armadale as a result of their proximity, a fate warned of in his father's letter. This feeling intensifies when the pair spend a night on a shipwreck off the Isle of Man—the very ship on which the murder was committed. On the vessel, Armadale has a mysterious dream involving three people; Midwinter believes that the events are a prophecy of the future.
Members of Armadale’s family die in mysterious circumstances. As a result, Armadale inherits the estate of Thorpe-Ambrose in Norfolk and relocates there with Midwinter, intending to make him his steward. Once there, Armadale falls in love with Miss Eleanor Milroy, the sixteen-year-old daughter of Major Milroy, to whom he has rented a cottage. Meanwhile, Miss Lydia Gwilt, the villainess of the novel, plots with Mrs. Maria Oldershaw for Miss Gwilt to marry Armadale and become the lady of Thorpe-Ambrose, as a means of enriching herself and achieving retribution for his family's apparent wrongdoings.
The colourful portrayal of the beautiful Lydia Gwilt takes up much of the rest of the story. Originally Armadale’s mother's maid, and a contributor to the conflict between Armadale’s and Midwinter's fathers, she is a fortune hunter and a poisoner. Unable to alienate Armadale’s affections from Miss Milroy, she next decides to marry Midwinter under his real name, which she has discovered is also Allan Armadale; she then plots to murder the wealthy Armadale, or have him killed by her ex-lover, a Cuban desperado; as the legal "Mrs. Allan Armadale", she plans to claim to be his widow, and the owner of Thorpe-Ambrose. However, Armadale escapes the desperado's attempt on his life—he is supposed to have drowned in a shipwreck—and returns to England. Lydia Gwilt's plans are thus foiled.
Her final plan is to murder Armadale herself, using poison gas, at the sanatorium run by a quack called Doctor Downward, but she is thwarted by her own conscience: Midwinter and Armadale have switched rooms, and she cannot bring herself to murder her true husband, for whom she has genuine feelings of love. After rescuing Midwinter and writing him a farewell note, she goes into the air-poisoned room and kills herself. Armadale marries Miss Milroy, and Midwinter, still his best friend, becomes a writer.
Some linking passages consist of letters between the various characters, or of extracts from Lydia Gwilt's diary, but the great majority of the text narrates the events as they occur. The novel is enlivened by many minor characters including Mr. Bashwood, an old failure of a clerk who is infatuated with Lydia Gwilt; his son, James "Jemmy" Bashwood, a private detective; Mrs. Oldershaw, an unscrupulous associate of Miss Gwilt’s; the Pedgifts (father and son), Armadale’s lawyers; and the Rev. Decimus Brock, a shrewd (but not quite shrewd enough) clergyman who brings Armadale up, but is absent for much of the book.