The novel has a convoluted plot about two distant cousins both named Allan Armadale. The father of one had murdered the father of the other (the two fathers are also named Allan Armadale). The story starts with a deathbed confession by the murderer in the form of a letter to be given to his baby son when he grows up. Many years pass. The son, mistreated at home, runs away from his mother and stepfather, and takes up a wandering life under the assumed name of Ozias Midwinter. He becomes a companion to his distant cousin, the other Allan Armadale, who throughout the novel never discovers the relationship. But Ozias is constantly haunted by feeling that he might harm Allan, first after he reads the letter left for him, and then again after they spend the night on a shipwreck off the Isle of Man-the ship turning out to be the same one which the murder took place (the murderer locked his victim in a cabin as the boat filled with water). On the boat, Allan has a mysterious dream involving three characters. This dream fills Ozias with foreboding, justifiably so as its three scenes become fulfilled in the course of the novel.Allan inherits estates at Thorpe Ambrose in Norfolk after the mysterious death of three of the family. He is unused to wealth, and falls in love with the sixteen-year-old daughter of Major Milroy, to whom he has rented a cottage. This love affair is for a period thwarted by the machinations of Miss Milroy's governess, Lydia Gwilt.Lydia, who is thirty-five but looks twenty-something, is the villain of the novel and her colourful portrayal takes up much of the rest of the story. Originally Allan's mother's maid, and a contributor to the conflict between Allan's and Ozias's fathers, she is a fortune-hunter and, it turns out, a murderess. Unable to alienate Allan's affections from Miss Milroy, she settles for marrying Midwinter, having discovered his name is the same. She plots to murder Allan-or to have him killed by her ex-husband, a Spanish desperado-and, since she is now "Mrs. Armadale," to impersonate his widow. Allan escapes the desperado's attempt on his life-he is supposed to have drowned in a shipwreck-and returns to England. Lydia's plans are thus foiled. Her last shot is to murder Allan herself-the weapon being poison gas, the scene being a sanatorium run by a quack called Doctor Downward-but she is thwarted by her own conscience. Midwinter and Allan have switched rooms, and she can't bring herself to murder her true husband, for whom she does have genuine feelings of love. After rescuing Midwinter and writing him a farewell note, she goes into the air-poisoned room and kills herself. Allan marries Miss Milroy; Midwinter, still his best friend, becomes a writer.Some linking passages consist of letters between the various characters, or of extracts from Lydia's journal, 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 the beautiful Lydia; his son, James Bashwood, a private detective; Mrs Oldershaw, an unscrupulous associate of Lydia's; the Pedgifts (father and son), Allan's sometime lawyers; and the Rev Decimus Brock, a shrewd (but not quite shrewd enough) clergymen who brings Allan up but who is kept out of the way for much of the book.Is the dream to be interpreted rationally or superstitiously, as Midwinter does? The question is never resolved."The distortions of the plot, the violent and irrational reactions of the characters, reflect and dramatise the ways in which his readers' perceptions were distorted by the assumptions and hypocrisies of the society in which they lived," writes Catherine Peters.In the end, the novel is a story of redemption that teaches that the sins of the fathers are not necessarily visited on the children, and the son of a murderer can turn out good. Collins was to take this up again later in The Legacy of Cain