The narrative follows an engineer summoned back to an abandoned coal field where an old foreman and his son have maintained secret life beneath the surface. Explorations reveal a self-contained subterranean settlement carved from exhausted galleries, with inventive measures for light, work, and social order; mysterious phenomena and experiments unsettle visitors; community bonds, trials such as accidents and structural hazards, and local legends shape daily life. A young girl taken into the community becomes central to domestic hopes and a later marriage, while revelations about the site's origins and a shadowy figure known as the Monk bring closure to the colony's fate.