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.