The lost city of Arkadia (named for Arcadia) resembles a small Alderson Disk, and is home to an ancient civilization which escaped a Great Cataclysm in the ancient past by relocating deep within the Earth's crust. Unaware that life continued on the Earth's surface, and hoping to keep their people safe, the elders sealed all records of their past in the city's Archives. Arkadia survives by the light of an artificial sun, the Tehra (Shagma), which is dying. A group of young Arkadian kids and teens defy the law and enter the Archives. With information about the world above, they create a messenger, Arkana, and send her above to find help. Arkana encounters two children from the surface, Matt and his sister Rebecca, and brings them back through the underground strata (which seem more like separate worlds or dimensions, with one strata even being the distant future) to save Arkadia. They travel in a living turtle type spaceship called Tehrig, along with Spartakus (a mysterious wanderer) and Bic and Bac (a pair of pangolin-like creatures), Arkadia's mascots.