Isaac Asimov’s The Golden Door, the last episode in the series, chronicles the history of the United States from the end of the American Civil War to the outbreak of World War I. Asimov explains the turbulent period of Reconstruction and the various challenges facing the nation, including the struggles for civil rights for newly freed slaves, the rise of corruption and industrialisation, and the emergence of a powerful nation on the global stage. The text further details the development of US foreign policy, examining key events such as the purchase of Alaska and the Spanish-American War. The author concludes by exploring the rise of progressivism and the challenges of integrating large numbers of immigrants into American society.