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 allow them to breathe underwater start to fail, thirteen-year-old Aluna and her friend Hoku go to that forbidden place to find help.

Aluna and Hoku, Kampii from the City of Shifting Tides, and their friends, Equian Dash and winged Aviar Calli, are determined to stop a war. The maniacal ex-scientist Karl Strand is planning to conquer the world with his enormous army of tech-enhanced soldiers-- unless the four friends can get to Strand first. Aluna's plan is dangerous: pose as Upgraders and infiltrate the army. But the enemy isn't what they expected and the strategy begins to crumble....