The Story Thus Far...

While you were off exploring space, an Alien Race has come and conquered Earth. They've killed off 90% of the population. You are certain your family is still alive. So you made the decision that rather than return to Earth to rescue your family, you're going to kill off all the Alien supply ships to deprive the Aliens of their critical supply of space toast. You don't know which Aliens are the ones who attacked Earth so you just have to kill them all. You don't know which ships are the supply ships either, so Good Luck. Once you have annihilated all invaders you'll be able to return home safely to Earth into the arms of your loving family.