The Arkadians are a race who have lived far beneath the Earth for thousands of years, tended by ancient technology and an artifical sun - but they have lost the knowledge of how their technology works, and their sun is failing. In desperation some of the younger citizens of Arkadia locate the ancient library and learn that other civilizations still exist on the Earth's surface. They create an 'avatar' of their race, named Arkana, who travels to the surface in the sentient ship Tehrig, along with animal pals Bic and Bac, to find someone who can understand the ancient technology. Who she finds are two human children, Matt and Rebecca, exploring some caves. Almost immediately they are attacked by Arkadian pirates, and descend into the Earth's interior after being saved by the final member of their crew, the mysterious stranger Spartakus.