"Miseinen dakedo Kodomo ja Nai" centers on the life of 14-year-old Kouta Hiiragi, a middle school student who struggles with societal expectations and the complexities of adolescence. Despite his young age, Kouta is often mistaken for an adult due to his tall stature and mature appearance, leading to frequent misunderstandings and awkward situations. This physical incongruity becomes a recurring theme as Kouta navigates daily life, grappling with the dissonance between how others perceive him and his actual emotional and psychological development.

The narrative explores Kouta's relationships with those around him, particularly his interactions with classmates, family, and strangers who often misjudge his age. His best friend, Yuuto, serves as a grounding presence, providing comic relief and a sense of normalcy amidst Kouta's struggles. Meanwhile, Kouta's older sister, Aoi, often teases him but also offers support, highlighting the familial dynamics that shape his experiences. The story also introduces a romantic subplot involving Kouta's crush on a classmate, further complicating his attempts to reconcile his outward appearance with his inner vulnerabilities.

Throughout the series, Kouta faces various challenges that stem from his unique predicament, such as being mistaken for a delinquent or being thrust into adult responsibilities. These situations often lead to humorous yet poignant moments, underscoring the tension between societal expectations and individual identity. The manga delves into themes of self-acceptance, the pressures of growing up, and the ways in which external perceptions can influence one's sense of self. By blending comedy with heartfelt moments, the series paints a relatable yet distinctive portrait of adolescence, capturing the awkwardness and resilience of youth.