A Lost Lady tells the story of Marian Forrester and her husband, Captain Daniel Forrester who live in the Western town of Sweet Water, along the Transcontinental Railroad. The story is told from the perspective of Niel Herbert, a young man who grows up in Sweet Water and witnesses the decline of Mrs. Forrester, for whom he feels very deeply, and also of the West itself from the idealized age of noble pioneers to the age of capitalist exploitation.