The chemical plague had swept through the city quicker then anyone could control. The humans tried, they fled from the worst of it but they left behind their pets. The loyal dogs that served them and loved them. These canines became wild, more violent than their domestic origins, some worse than others. Physically they remained the same, but their personalities had changed. Packs of dogs grew together, created gangs and took over the ruins of the city.
That was two years ago, now the humans are back and trying to reclaim the city. They've put up a fence to separate themselves from the worst of the abandoned city. But the fence is weak and the gangs are strong.