Kim Kang-woo was a poor farmer's son in a North Korean border town, top of his class but barred from university for being the wrong social class. It was pouring rain the night in May 2016 when Kim crossed the Yalu River alone. Construction sites, a mattress store, a cellphone store, every job he could land, thinking of his mother each night on the walk home. The day after she was detained, Kim crossed the Yalu a second time, without her. South Korean intelligence had tracked his whole journey by phone records and interrogated him for weeks as a suspected spy.