The zombies have taken over the world and now, the world is in total chaos, not only because of the zombies, but also because other people will simply kill you for supplies.  Stone and Marshall had banned together to overcome all odds. The game is played via mouse and you need to move those two one at a time.  Fight the soldiers or zombies that will be your problem. Finish the objective and you will get some cash, which you can use to upgrade.   Those upgrades will be useful in the long run because the challenges will be a lot tougher.