The zombies are here. No one knows where they came from, but it doesn't really matter. All that matters now is surviving until help comes. If help comes. You'd better hope that help comes.
The Shotgun Diaries uses a single d6 for resolution. This is only used when zombies are about and instead of indicating success or failure it selects whether the player or the GM does the narration. Other notable features are the Zombie Clock and the character journals.