The street is filled with the undead and most people are in panic nowadays. They don't know where to go and they don't know if they would live a little longer. Fortunately, a good Samaritan tries to help them by driving a bus around the city. Sure, his life would be in danger, but one thing is for sure, he is the light in the darkest day for the people that he is trying to save. There might not be anything for him for rescuing them, but the hell with that as times like this, we would be needing each other. There is an old saying that "one can't accomplish anything alone, but if there are more, anything can be accomplished."
The idea here is save as much people that you can. Sure, you don't know where to go to, but the idea here is to save people, so you need to explore the city till you find some survivors. You need to drop them off to the safe zone. The more the better for you, so you need to find more survivors since the score would be a lot better. The game gets harder and more complicated as you make some progress.