Fresh out of college, Kumiko Yamaguchi arrives at Shirokin Academy, an all-boys private high school, to fulfill her lifelong dream of becoming a teacher. However, her idealism is immediately tested when she is assigned as the homeroom teacher for class 3-D, a group of delinquents notorious for driving away their previous instructors. The students, led by the cool and observant Shin Sawada, initially try to make her quit through bullying and disrespect. Unbeknownst to the school faculty and her students, the cheerful and clumsy Kumiko is actually the heir to the Oedo Group, a powerful yakuza clan, and was raised by her grandfather, the clan's boss. Her upbringing has given her exceptional combat skills and a unique sense of personal honor.

Despite her intimidating background, Kumiko is a good-hearted young woman who truly believes in her students. She refuses to give up on class 3-D, using her unorthodox methods and surprising strength to protect them from expulsion and other external threats. Her persistence earns her the nickname Yankumi, a combination of yankee (delinquent) and sensei (teacher), a name she proudly embraces. The students gradually come to respect her, not knowing that their math teacher is secretly a yakuza princess.

The first season follows Yankumi as she navigates various crises with her class, including proving a student innocent of theft, confronting bullies from rival schools, resolving blackmail, and defending them against false accusations. A key narrative arc involves Shin Sawada accidentally discovering Yankumi's secret yakuza connection, which forces a deeper understanding between them. The season culminates in a final crisis where a magazine article exposes her background, causing a media scandal that forces her to resign to protect her students' futures and allow them to graduate. The success of the first season led to multiple sequels. Gokusen 2 follows Yankumi as she transfers to a new school, Kurogin Academy, where she once again takes charge of a class of delinquents. Gokusen 3 sees her return to the original Shirokin Academy to handle yet another problematic class, continuing her mission to guide troubled youth while hiding her true identity as the heir to a crime syndicate.