In 1980s Hong Kong, troubled youth Chan Lok-kwun, a mainland refugee, struggles to survive in the Kowloon Walled City by joining underground fights. Betrayed by crime boss Mr. Big while trying to buy a fake ID, he steals drugs from him and seeks refuge in the Walled City, where he encounters Cyclone, a compassionate yet authoritative crime lord.

YUM Investigation is a detective agency that specializes in the supernatural. With their new member Kojiro, the agency investigates a developer who dispatches evil spirits to scare away the residents of an old building. When they discover that the spirits were created by a sinister “spirit refiner”, the investigators must use every trick up their sleeves to crack the case.