It's hell out there, the zombies have overrun the planet. It is your job to survive them. Armed with a gun, blast your way through them and just make it to the safe zone indicated on your map by a green cross.
There are three strains on zombies observed out there. First are the yellows who deliver a powerful punch when they get close to you, second are the reds who cand deliver kicks twice as powerful and the blues who smell at a farther distance than any of the other two zombies.