Their former boss, the Korean-Chinese owner of an Internet sex operation, was hunting them as well. For the past year Red and White (aliases I gave them in my notebook in case police stopped me) had been held in a room as virtual prisoners, forced to "talk dirty" and take off their clothes in front of a camera for online clients in South Korea. The night before, Christian missionaries had helped them escape and brought them to this safe house.

The knocking continued. A man called out, "Are you there? Open up." White recognized the voice: It came from one of their rescuers. She rushed to the door and fumbled it open. Stand­ing there was a thin man with an awkward smile, holding up a cooker and a bag of rice. "You must be hungry." Bowing in greeting, the women led him into the kitchen. Soon the room filled with their chatter. The missionary also brought a message: "Be ready to leave soon. The call just came."

Some 50,000 North Koreans, and possibly many more, are hiding in China, most in cities and villages along the remote 900-mile-long border between the two countries. Uncounted others have come for a few months and then slipped back to North Korea with food and money. Yet many stay on, unable or unwilling to return to their cruel homeland. They are left with two desperate choices: Keep hiding—often as prisoners of exploitative employers—or embark on the Asian underground railroad, a perilous journey by foot, vehicle, and train across China and Southeast Asia. Confronted with an obstacle course of checkpoints, informants, and treacherous terrain, numerous defec­tors have been caught. But aided by a small band of humanitarians and by smugglers charging $3,000 and up, some 15,000 have reached safe haven, most often in South Korea. There, traumatized and barely skilled, they face the most formidable challenge of all: starting over.

The exodus from North Korea began in the mid-1990s as a devastating famine broke out across the country. In the worst hit areas, people were reduced to eating roots, grasses, and tree bark. More than 2.5 million people would perish. At first the Chinese openly aided the desperate border crossers. But following protests from the North Korean government, China cracked down. Police regularly raid neighborhoods and villages to ferret out North Korean runaways, who live in terror of being caught and deported. In North Korea, crossing the border without permission is punishable by three to five years in a prison labor camp, and conspiring with missionaries or others to reach South Korea is considered treason, with offenders starved, tortured, and sometimes publicly executed. Human rights organizations and various foreign leaders, particularly in the United States and the European Union, are urging China to honor its international agreements by treating the North Koreans as refugees, a status they're entitled to because of the punishments they face if deported. But China maintains that the defectors are illegal "economic migrants." In the months leading up to the 2008 Olympics, Chinese authorities intensified their efforts to apprehend defectors, capturing and deporting dozens, perhaps hundreds, a week. Yet they keep coming.