Jonathan Dark, a helicopter pilot transporting medical supplies in Southeast Asia, is forced down in the jungles of Cambodia, where he discovers Arangkhôr. There he slides into a well made of a mysteriously slippery substance, which proves to be a device of unknown provenance that teleports him to another world. The world in question is eventually determined to be the Jovian moon of Callisto, which beneath a projected illusion of airless desolation turns out to have a breathable atmosphere, an alien biology, and human inhabitants (presumably descended from victims of the well during the period before Arangkhôr was abandoned). Callisto is known to its inhabitants as Thanator.