Rumors claim that Juuzou Oogami, a middle-aged professional hitman, will assassinate anyone for a sufficient reward. However, this hearsay is far from the truth, as he adheres to a personal rule of only killing those who are truly evil and irredeemable. Because he began his killing career early, Oogami never experienced a normal childhood, and he regrets some of the choices he has made over the years. Oogami's life takes a drastic turn when, after another seemingly routine job, a mysterious wasp stings him with venom that reverts his body to that of a teenager! Although initially taken aback, Oogami soon receives a mission perfectly suited to his new form: he must blend in and investigate a private middle school, where his boss plans to enroll his daughter the following year. Despite failing to make a good first impression on his classmates, Oogami quickly grows to love schoolwork. Furthermore, he discovers that his classmate Noren Mitsuoka has connections to the company that created the very same wasp that stung him. As he befriends Noren and other students, Oogami sets out to enjoy the small aspects of being a teenager while searching for a way to restore his body.