The plot involves Kumiko Yamaguchi, the granddaughter of a Yakuza boss, Kuroda of the Kuroda Ikka. Her parents died when she was at the age of seven, and her grandfather had no other descendants, so Kumiko is next in line to head the family business with the title of Ojou. However, her lifelong dream has been to become a teacher. While her grandfather approves of her choice, others in the family want her to become the next boss. Kumiko (known by her students as "Yankumi") becomes a teacher at an all-boys private high school. Her class is full of delinquents, but she tries her hardest to teach them not just about academics (her subject is Mathematics), but about lessons of life, much like any "dedicated teacher" story. Though she's forced to keep her family a secret from the public, her Yakuza upbringing gives her the strength and the experience to reach out to her students, while also providing comedic relief.