"Granny Girl Hinata-chan" centers on Hinata, a high school girl who unexpectedly transforms into an elderly woman overnight. This sudden change disrupts her daily life, forcing her to navigate the challenges of maintaining her identity and relationships while grappling with her new appearance. Despite her physical transformation, Hinata retains her youthful personality, creating a stark contrast between her inner self and outward appearance. The story explores her efforts to adapt to this bizarre situation, often blending humor and heartfelt moments as she interacts with her classmates, family, and friends.

The narrative delves into themes of identity, self-acceptance, and societal perceptions of age and beauty. Hinata’s relationships with her peers, particularly her childhood friend Haruto, are central to the plot. Haruto, unaware of Hinata’s condition, struggles to reconcile his feelings for her with her elderly appearance, adding a layer of romantic tension. The series also introduces other characters, such as Hinata’s supportive but bewildered family and her classmates, who react in various ways to her transformation, ranging from confusion to acceptance.

As the story progresses, Hinata’s journey becomes one of resilience and growth. She learns to embrace her situation, finding strength in her ability to connect with others despite her altered appearance. The manga balances comedic elements, such as Hinata’s attempts to maintain her high school life, with poignant moments that highlight the emotional impact of her transformation. Through its unique premise, "Granny Girl Hinata-chan" offers a blend of humor, romance, and introspection, making it a distinctive addition to the slice-of-life genre.