A legendary hitman known for never failing a job, Juzo Ogami has spent his adult life as the Z.O.O. syndicate's most deadly operative. During a mission to eliminate criminals connected to the genetic manipulation company Mitsuoka Pharmaceuticals, he is stung by a mysterious wasp that rewrites his DNA and regresses his body to that of a thirteen-year-old boy. Unable to operate as an assassin in his new form, his boss gives him a strange new assignment: enroll in a private middle school to evaluate its safety for the boss's daughter.

Despite a lifetime of violence and no formal education, Ogami quickly discovers a genuine love for middle school life. He must now balance the chaos of classrooms, friendships, and extracurricular activities with his ongoing assassin work, all while keeping his true identity hidden from his classmates. His situation becomes even more complicated when he meets Noren Mitsuoka, the heiress of the pharmaceutical company that created the wasp that transformed him. Noren may hold the key to reversing his condition, but her family’s ties to dangerous genetic experiments force Ogami to navigate a web of corporate conspiracy and schoolyard drama.

Alongside classmates like the perceptive Tenma Tendo, the sporty Chisato Shiraishi, and the eccentric Kotatsu Nekota, Ogami experiences youthful milestones he never had—from futsal tournaments and midterms to school festivals and budding friendships. Each day pulls him further into the ordinary joys of adolescence, even as rival assassins, corporate agents, and the syndicate’s demands threaten to drag him back into the shadows. The central narrative arcs follow his struggle to protect his new life while hunting for a cure, forming genuine bonds that make him question the path he has walked for decades. As the secrets of Mitsuoka Pharmaceuticals unravel, Ogami must decide how much of his old self he is willing to leave behind.