From the back of the box:
RED ALERT! The four Capsuled Cities of Water Planet Hydron are under fierce enemy attack! You must pilot Nova 1, the last of the Novon Fleet, over turbulent seas. Blast Orion Fighters, Gravitines and six other types of airbone aliens before they obliterate the cities or damage your ship!
Beam energy aboard as you need it. Reinforce existing shields or beam new ones onto cities as theirs collapse! Use the Surface Scanner to locate and destroy enemies. Bomb Water Walkers before their rockets reach you. Repel repeated assaults! If you don't, no one will... ever.