(from: Wilkie Collins: An Illustrated Guide © Andrew Gasson 1998, used with permission)
In the Swiss town of Wildbad during 1832, Allan Armadale makes a death-bed confession of murder, written down by the only other British visitor, Mr Neal. The document is sent to Armadale's executors, to be given to his son when he comes of age, warning him never to contact people connected with the confession.
The dying man was born in Barbados and inherited the estate of his godfather, his father's cousin, on the condition that he changed his name to that of Allan Armadale, his godfather's disgraced natural son. After receiving the inheritance, Armadale had fallen in love with the portrait of Jane Blanchard, daughter of a family friend then staying in Madeira. But before he could sail to meet her, he was poisoned by his clerk, Fergus Ingleby. By the time he reached Jane she had married Ingleby, deceiving her father with a letter forged by her young maid. Ingleby was really the disgraced Allan Armadale, marrying Jane in revenge for the loss of his birthright. The married couple escaped on a timber-ship, ironically called La Grâce de Dieu, but Armadale followed them disguised as a crew member. The ship was partially wrecked during a hurricane and although Armadale saved Jane, he locked her husband in a cabin, leaving him to drown. Armadale departed for Trinidad where he married a half-caste woman who named their son Allan. Just before coming to Wildbad for his rapidly failing health, Armadale learned that Jane also had an infant son called Allan Armadale.
The main story begins in 1851 when Jane's son, Allan Armadale, has by a series of accidents inherited the Blanchard family estate at Thorpe Ambrose, Norfolk. In the intervening years he has been brought up in the West Country by his reclusive mother and the Revd Decimus Brock who knows there is an undisclosed family secret. Shortly after a visit by her former maid to extort money, Jane has died leaving Brock to look after her son. Allan, naïve and immature, takes everything at face value and acts totally on impulse.
Before he takes up his inheritance at Thorpe Ambrose, Allan arranges a sailing trip with his only friend, the dark and mysterious Ozias Midwinter. While cruising round the Isle of Man Midwinter receives the letter written by his father in Wildbad nineteen years earlier--he is in fact the other Allan Armadale in the second generation. He shows the document to Brock, also on the trip, and reveals the story of his earlier life. Midwinter had been ill-treated after his mother married Mr Neal. Escaping from school, he took up with a gipsy whose name he was happy to borrow. When the gipsy died, Midwinter continued the hard life of a vagabond, working as a servant, a seaman, and an usher in a school. Dismissed from this last post because of serious illness, he was rescued by Allan Armadale, the first person ever to treat him with kindness.
Midwinter burns the letter, pledging to keep its secrets from Allan, while Brock returns to Somerset now confident that Midwinter will protect his friend. They borrow a boat from the local doctor, Mr Hawbury, and become stranded onboard the nearby wreck of the old timber-ship, La Grâce de Dieu. Allan falls asleep and dreams of a drowning man, followed by three other visions: the shadow of a woman by a pool at sunset; the shadow of a man with a broken statue; and a man and woman passing him a glass, after which he faints. Midwinter is convinced the dream is supernatural while the doctor argues a rational explanation for all of the predictions.
Allan, now 22, takes up residence at Thorpe Ambrose and innocently upsets the entire neighbourhood as well as offending the family lawyer, Mr Darch. To replace him Allan engages Mr Pedgift and his son who recommend the elderly Mr Bashwood to teach Midwinter the duties of steward. Bashwood is experienced in this role but lost his previous position when bankrupted by a disreputable son, currently working as a private enquiry agent. The steward's cottage is occupied by Major Milroy, his bedridden wife and their daughter Eleanor (Neelie) Milroy with whom Allan immediately falls in love.
Jane Blanchard's former maid, the beautiful, red-headed Lydia Gwilt, hears about the inheritance from an old accomplice, Mrs Oldershaw, who encourages her to marry Allan for his money. Miss Gwilt, with the help of false references from Mrs Oldershaw, becomes governess to Miss Milroy. She first meets Allan at Hurle Mere on the Norfolk Broads and although Midwinter realizes she has fulfilled the first vision of the dream both he and Allan fall in love with her. Even the timid Bashwood is besotted with Miss Gwilt and agrees to act as her spy at Thorpe Ambrose.
Mrs Milroy is obsessively jealous of the governess and tricks Allan into making enquiries about Miss Gwilt's references. The resulting scandal puts him in an even worse light with his neighbours and Pedgift resigns when Allan declines to take his advice. Allan also quarrels with Midwinter and when a statuette is knocked over the second vision of the dream is fulfilled. Miss Gwilt realizes there is no chance of marrying Allan and when Midwinter reveals that his true name is also Allan Armadale she conceives a new plot. She has fallen in love with Midwinter, and if she marries him under his real name and can contrive that Allan leaves Thorpe Ambrose and should somehow die, she can return as his widow using the marriage certificate as evidence of her claim on the estate.
Allan goes to London to consult a new lawyer and Miss Gwilt arranges to travel in the same railway compartment. They are spotted by Bashwood, now so jealous that he employs his son to investigate Miss Gwilt's background. They learn that twenty-five years earlier she had been brought up by Mrs Oldershaw before Jane Blanchard took her abroad as a maid. To avoid further scandal after the death of Ingleby, the Blanchards sent her to be educated in France where she took up with a Russian card-sharper. She trapped a rich Englishman called Waldron into marriage but poisoned him when she was suspected of having an affair with a Cuban, Captain Manuel. She was tried and convicted of murder but the case caused such a public outcry that the sentence was quashed and she served just two years in prison for theft. On release, the Cuban married her bigamously but abandoned her after spending the money from Waldron's will. She returned to Mrs Oldershaw who ran a beauty parlour in association with the shady Dr Downward.
Back in London, Miss Gwilt reconciles Midwinter and Allan and secretly marries Midwinter. Needing Allan to keep away from Thorpe Ambrose, she encourages him to see Brock in Somerset and then to visit Naples where she will be living with her husband. Two months later Allan arrives in Italy and one evening after the opera faints when given a drink, so fulfilling the third vision from the dream. Miss Gwilt is approached at the same time by Manuel who tries to blackmail her for money. Instead she arranges for him to captain Allan's new yacht, knowing that the Cuban will rob and kill him.
Miss Gwilt returns to London and enlists the help of Dr Downward. Under the alias Le Doux, he is opening a private sanatorium in Hampstead where she pretends to be a patient in order to avoid Midwinter. Miss Gwilt sees a newspaper report that Allan's yacht has been sunk and writes to Thorpe Ambrose as his widow. She once again makes use of Bashwood and he informs her that Allan has been miraculously saved and is on his way back to England. Late at night, under the pretext of seeing Miss Milroy, Bashwood lures Allan and Midwinter to the sanatorium where the doctor offers them accommodation. Downward has shown Miss Gwilt the means of killing Allan by filling his room with a deadly gas which leaves no trace; he will seem to have died from natural causes. Midwinter senses that something is wrong and finding an excuse to change rooms is overcome by the gas. Miss Gwilt suddenly realizes her mistake. She still loves Midwinter and drags his unconscious body into the corridor. When she is certain he will live, she writes a confession before killing herself with the poison.
Midwinter allows the world to think Miss Gwilt died naturally while Bashwood loses his reason. Allan marries Miss Milroy leaving Midwinter convinced that the visions were in fact meant to bring the two Armadales closer together.