A revealing look at the Chinese government’s mass imprisonment of an estimated two million Uyghurs and other Muslims, with undercover footage inside China's secretive Xinjiang region. Experts have described it as the largest mass incarceration of an ethnic group since the Holocaust. Now, with undercover footage and firsthand accounts from survivors of China's detention camps, FRONTLINE investigates the Chinese Communist regime’s detention of Muslims — and its use and testing of sophisticated surveillance technology against the Uyghur community.