When fifteen-year-old Ty, who has always lived on the ocean floor, joins Topside girl Gemma in the frontier's underworld to seek and stop outlaws who threaten his home, they learn that the government may pose an even greater threat.

In a future of high technology and genetic modification the Coral Kampii, like legendary mermaids, live isolated from the Above World, but when the devices that allows them to breathe underwater start to fail, thirteen-year-old Aluna and her friend Hoku go to that forbidden place to find help