For the first time ever most of the world's people are now living in cities, but as the ranks of the world’s cities swell many are literally falling through the cracks. In Mexico City, one of the planet’s largest urban areas, there are thousands of young people living in gangs, beneath highway overpasses or hidden in makeshift subterranean shelters. This is the story of one group, the "Mushroom Roundabout Gang", and of their struggle for survival in the face of parental abuse, violence, and drugs.