Candlewick Press.

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....