The story of a young, idealistic teacher who enters Shirokin Gakuen with hopes of making a difference in the students' lives. However, Yamaguchi Kumiko (Nakama Yukie) is in for a rude awakening when she finds that she will be the homeroom teacher of 3-D, the hardest class to maintain in the school. The students have multi-colored hair, sloppy uniforms, and show absolutely no respect to her. The leader of the class, Sawada Shin (Matsumoto Jun) is lazy but bright and shows some interest in Yamaguchi when he notices that she is very different from other teachers. The students try to bully her to make her quit, but their attempts don't get them very far.
What the class and the rest of the school does not know is that Yamaguchi Kumiko is actually the 4th generation heir to the Ooedo Group, a notorious yakuza clan. Even though her family would like her to take the title of Ojou when her grandfather dies, they have accepted the fact that Yamaguchi would rather be a teacher.
Eventually, the students learn to respect their quirky teacher because of her devotion to them, and even give her a nickname: Yankumi.
* Nakama Yukie as Yamaguchi Kumiko (Yankumi / Ojou / Koumichou) (Math Teacher/Class 3D adviser)
* Matsumoto Jun as Sawada Shin
* Namase Katsuhisa as Sawatari Goro, Kyoto (School Vice Principal)
* Utsui Ken as Kuroda Ryuichiro (Kumiko's grandpa / 3rd Generation Oedo Yakuza Boss)
* Anan Kenji as Wakamatsu Hiro
* Kaneko Ken as Asakura Tetsu
* Uchiyama Shinji (内山信二) as Tatsukawa Minoru
* Ryogoku Hiroshi (両國宏) as Sugawara Makoto
* Oguri Shun as "Uchi" / Uchiyama Haruhiko
* Waki Tomohiro as "Kuma" / Kumai Teruo
* Narimiya Hiroki as Noda Takeshi
* Ishigaki Yuma as Minami Youichi
* Itoh Misaki as Fujiyama Shizuka (English Teacher)
* Nakazawa Yuko as Kawashima Kikuno (School Nurse)
* Nabe Osami as Ando Osamu (Social Studies Teacher)
* Kohmoto Masahiro as Iwamoto Kouhei (P.E. Teacher)
* Ookuma Ichirou (大隈いちろう) as Oyama Hajime (Science Teacher)
* Tayama Ryosei as Shirakawa Gonzou (Principal)
* Saito Satoru as Washio Kanji (Japanese Language Teacher)
* Sawamura Ikki as Shinohara Tomoya (Police Officer)
* Sakata Tadashi (坂田聡) as Kashiwagi Yutaka (Police Officer)