How about this?
A biological attack on New York City wipes out 99.987% of the population. After surviving the attack, a young woman is hunted by giant mechs. She is helped by an alcoholic homeless dude who seems to know more than he should about the bio attack and the mechs. She leads a small band of survivors out of the city to safety in New Jersey only to find that she must return to the devastated New York in order to do something which is critically important but I haven't figured out yet.