The story of Gokusen begins as Kumiko starts her first teaching job at an all-boys' high school. Unfortunately, she is assigned to become the homeroom teacher for class 3-D, which is filled with delinquent and dangerous students. Other teachers warn her about their behavior and to be especially careful of the class leader, Shin Sawada, who was expelled from junior high school for punching a teacher. But, Kumiko is determined to win their respect and become the motivational teacher she has always dreamed of being. However, her glasses, childish hairstyle, and out-of-date appearance lead the students of 3-D to believe she will be an easy target for teasing. Despite the class's attempts to get rid of her, Kumiko still has no intention of quitting just yet.

Outside of school, Kumiko also has a secret life that her students nor co-workers are aware of. She is actually a fourth generation family member and heir to the Ohedo Ikka (Kuroda Ikka in the manga version), a powerful yakuza group in Tokyo. Kumiko's parents died when she was just in elementary school and moved into the Ohedo clan since her grandfather was her only living relative. Influenced by those around her, she grew up learning how to fight, maintain a tough attitude, and to speak with a rough tongue. But Kumiko wanted to fulfill her father's dream of becoming a teacher. However, due to school district and PTA regulations, she must keep the fact that she has any ties to the yakuza underground a secret.

Expecting Kumiko to cry and run away, the class leader, Shin, and the other students in 3-D realize that she won't be an easy teacher to get rid of. Kumiko's fighting skills and "yakuza-style" mannerisms are revealed through her natural personality, although she desperately tries to conceal them. However, it is with the skills she learned in the yakuza world that enable her teach her students. Whether it's literally knocking some sense into her students or physically battling those that threaten to harm them, she is willing to stake everything for them. Although Kumiko remains a mystery to her students, they slowly begin to open up and respect her.

Gokusen has a line-up of popular Japanese actors and actresses such as Yukie Nakama, (Ring 0 Birthday) who has been praised for her performance as the lead role of the yakuza teacher in Gokusen. Arashi pop idol Jun Matsumoto (New Kindaichi Shonen no Jinkembo) portrayed the delinquent class leader, Shin Sawada, and ex-Morning Musume leader Yuko Nakazawa made her appearance as the school nurse in the series. Even though these celebrities are not new the world of Japanese entertainment, the success of Gokusen seemed to paved the way to even more stardom.

Although the series ended last year, the popularity of the series inspired an additional special episode, Gokusen Returns, which aired this past December. Unlike other popular Japanese live-action TV series about teachers and students, Gokusen is based upon an original manga series. Therefore it is questioned if it will follow the same route as the manga of GTO, which spawned from a live-action TV series to an animated series and theatrical movie release. However, Gokusen's story differs from GTO in its own unique way that contributes to the success of the series. Unlike Onizuka, Kumiko does not appear to be strong or "cool" and hides the fact that she has ties to the yakuza underground. She uses her own strength and ideals to discipline her resistant students. Also, while Onizuka's physical battles were replaced with sarcastic wit and humor in the live-action version, Kumiko's character has to whip out all the kicks and punches which adds more excitement to the series. Add in humor and a romance plot between Kumiko and a police officer (the yakuza's "enemy") to Gokusen, and viewers can see that this storyline has a little for everyone.