In this game you're an alien who tries to destroy everything that the human beings have built on planet earth. You have several weapons at your disposal: the more points you earn, the bigger the choice of weapons. The green meter indicates how much ammo you have left: if the level's too low, it's temporarily impossible to use your weapon. If you click on the green button, a map will appear, with all the areas on it that you can choose. Click on the blue 'Achievements' button to see which goals you have to achieve.