When a mysterious murder happens - a teacher is killed inside the classroom - and there are not many clues, investigator Mi Nan calls on Fang Mu, a forensic expert with great deductive power, to help with the case. A second homicide happens, now a woman is killed in a arson, and the investigation points to the work of a self-styled serial killer who calls herself "the city light" and is attacking people who committed crimes and managed to escape conviction because of loopholes in the law. The killer starts a cat and mouse game with Fang.
