basically it's another normal day in NY city, when all of a sudden...
*Spackle!!*
Something crashes into the Statue of Liberty, taking off the poor green woman's head. Then...
*Holy Chimp Sauce Batman!!*
That something begins attacking the city and making lots of little somethings, and some teens are trying to get out of the city through either a) the tunnels b) the roads, or c) dying and having their corpses moved out of the city.