At 23 years of age, Kumiko Yamaguchi had finally achieved her childhood dream of being a teacher. Unfortunately, the homeroom she teaches math in at Shirokin Gakuen Private Boys' High School happens to be filled with a group of juvenile delinquents (like the rest of the school), which is led by the enigmatic Shin Sawada, and has to struggle in order to get them to take their lessons seriously. If the kids weren't bad enough, the school's vice principle, Goro Sawatari, is almost hoping Kumiko will fail at her job, sometimes sabotaging her efforts.
If everyone knew what Kumiko's family history was, they might be a little careful around her. You see, Kumiko is also the acting Koumichou, or head of the Ooedo Clan, a powerful yakuza group. Her grandfather Ryuichiro Kuroda, the Koumichou of the Ooedo and guardian of Kumiko since her parents' deaths, had fallen ill and now she has to fill-in for him. The group of hoods that often surrounds Kumiko (who addresses her as 'Ojou'), which includes Tetsu Asakura and Minoru Tatsuwaka, tend to be a bit overprotective of Kumiko, and she has to assert her authority over these lunks to keep them in line... especially when they heard the students made fun of her during her first day in class!
Kumiko is an excellent martial artist, which came in handy when she found out that a former student of the school, Hiroki Kudoh, had sneaked back in to beat-up the juniors of the school – which included Kuma. However, Kumiko has to be very careful not to let the secret that she is the Koumichou of the Ooedo slip. However, Shin suspects that there is something different about Kumiko, so she has to be careful around him.