Shiba is a detective in the fifth anti-organized crime division of the Tokyo Metropolitan Police, a.k.a. "Anti-Gang 5" specializing in handgun seizures. Shiba’s secret weapon is "S" ("spy"), an informant who plays a key role in Shiba’s information gathering. Shiba is training an informant who has been officially accepted into Shinjuku’s Matsukura Group within the Budoha crime syndicate. One day, Shiba awakens to an anonymous phone call - "Watch out for your S!"