Atsushi Miyagawa is a fifth-grader trapped in the body of a fully grown adult, his behavior and mental development perfectly matching his actual age. This jarring contradiction spawns constant misunderstandings. Ordinary childhood activities—like holding hands with a friend or playing outside—routinely trigger police intervention, as officers mistake him for a threat. These incidents plague his life multiple times weekly, though their frequency lessens as some law enforcement become familiar with his situation. His crush, and eventual girlfriend, classmate Hina Soejima, is often at the center of these public alarms.

His family dynamic adds another layer of confusion. His older sister, high schooler Atsumi Miyagawa, appears significantly younger than her age, making the siblings seem visually reversed. Atsumi acts protectively, advising him on clothing choices and intervening during problematic encounters. Despite her efforts, Atsushi's limited grasp of social perception frequently draws unwanted public attention. He feels the sting of societal treatment, like being denied sweets offered to his sister or facing suspicion during innocent moments.

Relationships further define his world. Elementary school teacher Futami Moriyama struggles to interact with him, uncomfortable around adult-looking males despite knowing his true age, though she occasionally helps during legal issues. Classmates like Hina treat him as a peer. Neighbor "Big Bro Take," an unemployed man in his 30s, sometimes gifts him age-inappropriate clothing like fishnet tops, worsening misunderstandings. Others, like Sayo Takahashi, develop romantic interest before learning his real age. Police officers Miura, Kijima, and Aono gradually recognize his circumstances after repeated encounters.

School events highlight his struggles: his physical prowess accidentally injures others during activities like three-legged races at festivals, or he's mistaken for a parent in parent-child events. Despite these challenges, he slowly adapts. Community members eventually acknowledge his childlike nature during neighborhood play. Throughout his experiences, his core personality remains consistently innocent and age-appropriate, focused solely on typical childhood interests and relationships without significant maturation.