Those damn scientists will never learn. The survivors you rescued last time have been messing around with the genetic code and have created a new and powerful breed of space mutants. The invaders have started infiltrating our Trebulon 6 colony and you have to stop them. It seems the scientists were the first ones to perish so just clean the place up, for as long as you can. There will be number waves of mutants attacking you and surviving there will really be a tough task for anyone. However, you will need to be speedy and accurate enough in shooting. As the game advances, more enemies will come out and you will find it harder to survive.