The manga centers around Ginjirou Manda, a loan shark business man who is affiliated with many Yakuza gangs in Osaka. He charges an interest rate called toichi — 10% more interest every 10 days, or 365% every year. Thanks to his unrelenting money-collecting tactics, he is feared as "Minami no Oni" or the "Demon" of Osaka's Minami business district. Even when his borrowers tries to scam him, he always finds a way to make them pay his money back. (Source: Anime News Network)

The story begins on Hell Island, the only remaining penal island in the modern era, where the legendary yakuza convict Rikido Omasa is granted parole after serving 30 years. He is told by Inspector Jirocho Shimizu of the Ominami Police Station—the man who once arrested him—that this is a case of “using poison to fight poison”: a prisoner, a “deadly poison,” will become a detective to suppress the “poison” of crime that infests modern society…

Volume 1 was authored by MATSUDA Yasushi. Volumes 2 & 3 was authored by KYONO Ichiro.