The Mark of Cain by Andrew Lang is a mystery novel about hidden guilt, suspicious death, and the secrets that connect people across class and society. The story begins with Maitland, a wealthy man involved in social reform, whose world is disturbed when a former acquaintance is found dead. As the mystery unfolds, the novel explores crime, identity, moral responsibility, and the consequences of past actions. Lang combines suspense with social observation, creating a story of intrigue, deception, and justice.