There are 130 million migrant workers in China. Many of them are young women who work illegally long hours for illegally low pay in colossal factories that are self-contained worlds unto themselves, with their own dorms and cafeterias and movie theaters and hospitals. Chang, a journalist, followed the lives of several of these factory girls, many of whom fled even grimmer situations in rural villages. The only traces we in the West see of this shadowy world are the cheap export goods the girls create, but it's a vast and complex society with its own iron laws, and it holds vital clues to the future of China and of the larger global economy it services.