Fresh out of college, Kumiko Yamaguchi has achieved her dream of becoming a teacher at Shirokin High School. She is assigned as the homeroom teacher for class 3-D, which is infamous as the dumping ground for the school's most hopeless delinquents. Her students, led by the cool and intelligent Shin Sawada, immediately try to make her life a living hell, expecting her to quit like all their previous teachers. What they do not know is that Kumiko, who they nickname Yankumi, is the heiress to the Oedo Group, a powerful yakuza clan. Raised by her gang boss grandfather, she possesses formidable martial arts skills and a strong sense of loyalty and justice derived from the yakuza code.

The central conflict of the first season revolves around Yankumi’s struggle to keep her secret identity hidden from the school faculty, particularly the scheming vice-principal Goro Sawatari, while using her unconventional methods to protect her students. Her grandfather Ryuichiro Kuroda and the Oedo family enforcer Tatsurou Asakura provide both comedic relief and occasional backup. Throughout the series, Yankumi’s sincere dedication breaks through the students’ hardened exteriors. In one early arc, she risks her career to prove that student Teruo Kumai, known as Kuma, was framed for theft, earning the class’s initial respect. As the season progresses, she helps her students navigate a series of problems including gang fights, bullying, and conflicts with parents.

The narrative builds to a climax when a reporter discovers Yankumi’s yakuza lineage and exposes it in a magazine, causing a scandal that forces the school to demand her resignation. In the final episodes, class 3-D, having come to truly respect her, rebels against the school administration and the media. To save her students from being expelled and allow them to graduate, Yankumi makes the difficult decision to resign. The season concludes with her former students successfully graduating, while Yankumi moves on to a new teaching position, ready to start the cycle again. The success of the first season led to two additional seasons and a feature film, with Nakama reprising her role as Yankumi, who continues to be assigned to the most problematic classes at different schools.