Somehow, the world, or at least the city you are in, has had its inhabitants slaughtered and resurrected with a hunger for brains, or their murderers have minions trying to find you and any accomplices.
Your goal: Don't die before help arrives or before you reach an exit. You will have close escapes from horrible creatures. Things will jump through windows at you. Sometimes, you will be forced to fight the horrible creatures or flee for your life. Other times you will hide in a shadowy corner praying the invincible monstrosity doesn't notice you as it lumbers past. Ultimately, you tend to be the only one who survives as anybody who could help you will die a horrible death-by-cutscene.