The survivors are holding out in an abandoned building with a few other uninfected. A fire breaks out, forcing them out the back of the building and away from the others, and also into the waiting hordes ambush. They're swarmed and barely make it out alive. Now alone and out in the open, they desperately start searching for safety and a way out of the city.
Screenshot showing the compromised saferoom in Map 1, from v.04 of the campaign.