After receiving a fragmentary manuscript describing a vanished explorer's entry into a secret cavern, a determined young woman organizes an expedition into the African interior to locate the lost passage and its author. The party discovers an isolated, highly civilized city hidden within mountain caverns whose inhabitants observe a secret sacred day and follow unfamiliar laws. The narrative traces their passage across subterranean bridges and through ice caves, sea-monster encounters, earthquakes, and death-traps, while political accusations, priestly authority, and a developing romance complicate efforts to rescue and escape. Perils such as burial, starvation, battles, and natural cataclysms culminate in a final confrontation tied to the community's sacred observance and the survivors' return.