"Tonari no 3 Shimai" centers on the lives of three sisters—Miu, Mai, and Mii—who live together after the death of their parents. The eldest, Miu, takes on the responsibility of caring for her younger siblings, balancing her role as a surrogate parent with her own personal struggles. Mai, the middle sister, is energetic and outgoing, often acting as a bridge between the serious Miu and the youngest, Mii, who is shy and introverted. The narrative explores their daily lives, focusing on their relationships, challenges, and growth as they navigate school, friendships, and the complexities of adolescence.

The story unfolds in a slice-of-life format, emphasizing the sisters' interactions with each other and their community. Miu’s dedication to her siblings often leads to moments of tension, as she grapples with the weight of her responsibilities while trying to maintain her own identity. Mai’s cheerful demeanor masks her own insecurities, particularly her desire to be seen as more than just the "middle sister." Mii, though quiet, gradually opens up, revealing her deep affection for her sisters and her struggle to find her place in the world.

Throughout the series, the sisters encounter various supporting characters, including classmates, neighbors, and family friends, who influence their lives in different ways. These interactions highlight themes of family, resilience, and the importance of communication. The manga avoids melodrama, instead opting for a grounded portrayal of the sisters’ lives, with moments of humor, warmth, and poignancy. The narrative’s strength lies in its ability to capture the nuances of sibling dynamics and the quiet, everyday moments that shape their bond.