Juzo Ogami is a legendary 39-year-old hitman with a perfect record, working for the Z.O.O. syndicate. His life takes an unexpected turn during a mission against a genetic manipulation company when he is stung by a mysterious, genetically-modified wasp. The sting triggers a bizarre reaction, transforming his body into that of a 13-year-old boy. Now trapped in a child's form, he is given a new assignment by his boss: infiltrate a middle school. His mission is to assess the safety of the school that the boss's daughter will be attending, while the syndicate works on finding an antidote.

The setting shifts between the shadowy world of professional assassins and the brightly lit halls of an ordinary Japanese middle school. Despite his initial reluctance, Juzo, who dropped out of school long ago, discovers an unexpected and deep love for learning. He becomes a model student, fascinated by subjects like negative numbers and social studies, which forces him to balance his deadly instincts with the mundane realities of homework, classmates, and gym class.

Key characters surrounding Juzo include his quirky and tech-savvy young partner, Kotatsu Nekota, who provides him with gadget support from afar. At school, he encounters Noren Mitsuoka, the strong-willed daughter of the pharmaceutical empire that created the wasp, making her a potential key to reversing his condition. Other notable figures include Chisato Shiraishi, the head of the home economics club, and several dangerous adversaries who also infiltrate the school, such as Shin Kohazame, an operative from a Chinese assassin organization, and the Rindou brothers, Kazuma and Eiji, who are members of a powerful assassin group.

As Juzo navigates his double life, he finds his past catching up with him. Rival assassins close in, turning the school into a battleground where he must use his adult经验和 lethal skills to protect his new cover and his classmates, all while searching for a way to return to his original body. The narrative arc follows his struggle to reconcile his identity as a cold-blooded killer with the rediscovered joys and social connections of adolescence, often using his mature perspective to solve problems, such as when he instinctively defends female classmates from a predator because they are the same age as his daughter.