From the introduction:
You don’t know why, but you know that shambling, formerly human creatures have invaded your peaceful little suburb. As far as you can tell, everyone else in the city is dead, except for a terrified few other that you have found. You have all congregated together for mutual protection, but as the stress increases, the internal tensions of your compatriots may be more dangerous to you than the undead outside the window.
Thematically, the game is about the breakdown of social taboos and mores. Once these outside limitations are removed, the hatred that usually bubbles beneath the surface comes forth. Zombies are only in the game peripherally as a way of addressing these themes, really. Perhaps other apocalypses could be substituted. Anything where the normal rules of society have broken down. The nice suburb that the PCs lived in has been disrupted, for some reason, and now they have to band together and deal with that. More frightening, though, is that they need to deal with each other as their own sense of morality breaks down.