Htrae is like Earth in many ways: oxygen, water, soil, and humans. The citizens of Htrae have elemental powers which they incorporate into the overall function of their society. Your elemental power determines not only your career but your status within Htrae. Pure powers, half powers and Zeros (no powers) make up the citizen population.
Earth has been watching Htrae and invades the planet with weapons the Pures and Halves have never seen before. Their powers are of no match. It all comes down the Zeros to stop the invasion of Htrae with the weapons they have been designing for years to become equal with their planet’s upper class.