Fresh out of college, Kumiko Yamaguchi fulfills her lifelong dream of becoming a teacher at Shirokin High School. However, her idealism is immediately tested when she is assigned as the homeroom teacher for class 3-D, the school's notorious dumping ground for delinquents. The students, led by the cool and intelligent Shin Sawada, initially try to bully her into quitting. To their surprise, their new teacher is utterly unshakeable. The reason for her resilience is a closely guarded secret: Kumiko is the heiress to the Oedo Group, one of Tokyo's most powerful yakuza clans. Raised by her grandfather, the clan boss, she possesses formidable combat skills and an unorthodox sense of morality drawn from the yakuza code of honor.

Despite her fierce appearance and tendency to use gangster slang, Kumiko, who the students nickname Yankumi, is a kind-hearted and devoted teacher. She uses her yakuza-honed instincts to protect her students from expulsion, rival gangs, and their own self-destructive tendencies, all while trying to keep her family background a secret from the school's staff, especially the scheming vice-principal, Goro Sawatari.

The first season focuses on Yankumi earning the trust of her five core students. The leader Shin is the first to discover her secret when he follows her home and meets her grandfather. The other students include the quick-tempered Youichi Minami, the blonde-haired Haruhiko Uchiyama, the gentle giant Teruo Kumai, and the eccentric Takeshi Noda, who gives her the nickname Yankumi. Throughout the school year, Yankumi tackles issues ranging from accusations of theft and bullying to fighting for a fellow teachers job and saving students from loan sharks. In each situation, she proves that loyalty and protecting ones family, whether blood-related or found in the classroom, are the most important values.

The narrative arc culminates when a reporter discovers her yakuza lineage, leading to a media scandal. The school board forces Yankumi to resign, and when the students violently confront the press in her defense, they face mass expulsion, jeopardizing their graduation. To save her students futures, Yankumi agrees to leave Shirokin. In a final act of defiance and love, her class refuses to let her go without a fight, leading to a dramatic confrontation where the students and their yakuza heiress teacher prove that they have, against all odds, become a family.

The dramas success led to two sequel seasons, Gokusen 2 and Gokusen 3, and a concluding theatrical film. In these sequels, a now more experienced Yankumi transfers to a new all-boys high school, once again finding herself in charge of a class of hopeless delinquents. While the supporting cast changes, the formula remains consistent: Yankumi uses her fists and her heart to reform her students, with occasional cameos from beloved characters from the first season, such as the now-married Kuma, who acts as a mentor to the new generation.