A modern city with a developed economy but complicated social contradictions. Under the bright surface of high-rise buildings, there are undercurrents in a gray area that cannot be touched by the law. Surveillance coverage in the city is as high as 75%, and facial recognition technology is widely used. However, it is in such a "transparent society" that a mysterious serial killer can easily shuttle between the gap between law and technology, creating a series of chilling and perfect crimes. His chosen targets are those who have committed serious crimes (rape, murder, corruption, drug trafficking, etc.) But are getting away with it due to lack of evidence or loopholes in the law. Before each case, he would anonymously send a "trial letter" detailing the target's crimes and evidence - which was often more comprehensive than what the police had.
