In a galaxy far away all the planets use Xnergy as their primary energy source. The Xnergy can only be collected on the planet Enerton, but this has become a dangerous job. Lured by all the collectors working on the planet, monsters have come.
For protection they have installed weapons on the working stations, but these weapons also need Xnergy. Because of this the Xnergy reserves on the planets are running low.
You must collect the Xnergy to keep your own weapons working, but also collect enough for the planets. You will get money for the Xnergy you collect for the planets and with the money you can buy new weapons. Every level gets harder as more monsters and different monsters will spawn. The game is all about making the right decision about when to go back to the safe tower or get some extra Xnergy.