If the game freezes, just click inside it (games can get unresponsive if you click elsewhere on the page).
Earth has been taken by an evil alien race. Your job is to destroy them one by one as you travel from city to city. Make it to your safehouse for decontamination or die trying. Fin other survivors that will help you along the way. Find ammo and food rations you can use yourself or sell them in the shop for other items.