Stepping into the van guarded by armed soldiers with five surgeons and nurses, Zheng Zhi didn’t know he was entering into a world that would haunt him for the next quarter of a century. Dr. Zheng, then a resident doctor at one of China’s largest military hospitals, knew little more than they were on a “secret military mission” near a military prison located around the northeastern Chinese city of Dalian. A light blue fabric covered the four sides of the vehicle, shielding it from any curious glances. When the door opened, four burly soldiers carried in a man whose limbs...