Rumors say that Juuzou Oogami, a middle-aged, professional hitman, will assassinate anyone for an adequate reward. The hearsay could not be further away from the truth, though, as he has a personal rule to only kill those who are truly evil and irredeemable. But due to starting his killing career early, Oogami never experienced a normal childhood, and he regrets some of the choices he has made over the years.

However, Oogami's life takes a drastic turn when—after another seemingly regular job—a mysterious wasp stings him with venom which reverts his body to the form of a teenager! Although taken aback, Oogami soon receives a mission that is perfect for his new body: he needs to blend into and investigate a private middle school, where his boss is enrolling his daughter next year.

Despite failing to leave a good first impression on his classmates, Oogami quickly falls in love with schoolwork. Moreover, 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 little things about being a teenager while searching for a way to restore his body.