According to the premise of Destroy All Humans!, the latest game from Pandemic, the developers of Star Wars: Battlefront and Mercenaries, the human race is under siege by the Furons, a race of highly-developed little gray men, intent on bending humanity to their will and sucking out our precious brain DNA for use in their cloning procedures. Given that the human race is likely to mobilize to stop the incipient invasion, it does seem odd that the Furons can't manage to muster more than a single dude to send down to the planet, but it all seems to work out ok, due to their ability to clone him whenever he dies. That, and their cache of advanced weaponry, should be enough to conquer the planet and absorb all of humanity's sweet, sweet cranial juices.
Of course, it'll help if you have our walkthrough for the game. We've got the rundown on each and every mission in the game, including the side missions in each of the game's six areas. Note that we're not including the locations of the many hidden Alien Probes in this guide, as all they really do is unlock more concept art.