Shiki, an academically gifted but socially timid high school student, is forced to enroll at St. Lukeman's Academy, a notorious school overrun with delinquents. His good-natured personality makes him an easy target, and he is relentlessly bullied by classmates led by the thug Kawatou, enduring daily beatings and errand-running to survive. One Saturday, Shiki witnesses Kawatou's gang harassing a younger girl named Haruno Momozono with an aggressive, near-kidnapping-style approach. Though terrified, Shiki intervenes to protect her and is badly beaten for his trouble. Just as the situation escalates, he is rescued by Karin Kohinata, a legendary figure known as the Empress, who is both the most beautiful and the strongest brawler in the school. Haruno is her friend. Impressed by Shiki's courage, Karin offers a deal: she will teach him how to fight, and in return, he will tutor her and her friends in their studies. This arrangement transforms Shiki's after-school life entirely.

Shiki becomes a regular fixture in Karin's circle, receiving intensive fighting lessons from her and her fellow gal delinquents, each with their own unique and often embarrassing teaching methods. As he grows stronger, he gradually sheds his victim status and gains respect from the school's underworld. The narrative follows Shiki's rise from the bottom to the top of the school's violent hierarchy. He defeats Kawatou in a one-on-one fight, establishing himself as a credible force. This victory draws the attention of larger factions. The first major arc involves the Arai faction, the dominant gang in the school, which targets Karin's group by exploiting a weakness. Shiki and Karin's crew must fight to protect their place and their friends. After toppling the Arai leadership, Shiki gains notoriety but also becomes a constant target for challengers. This leads directly into the second major arc: the First-Year Strongest Decisive Battle, a brutal battle royale hosted by the dangerous upperclassman beauty Akuri Kitou in an abandoned hospital. Shiki is forced to enter the tournament to protect his group and must teach a reluctant Haruno how to fight as she is also dragged into the competition. The final major arc escalates the conflict beyond the school gates. Shiki's success in the underground fighting scene makes him the target of a semi-criminal organization called Aurum, which threatens Karin's faction and the entire school. To defend everyone, Shiki must reach his peak strength and confront the organization in an all-out gang war, culminating in a climactic battle that decides the fate of the school. The series ends with Shiki having fully ascended to the top, having found strength, love, and a place where he belongs.