The novel follows two distant cousins both named Allan Armadale. The father of one had murdered the father of the other. 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.