"Haruba-Ke no Sanninme" centers on the Haruba family, a seemingly ordinary household with a unique dynamic shaped by the presence of three siblings. The eldest, Haruba Kei, is a responsible and pragmatic individual who often takes on the role of caretaker for his younger siblings. The middle child, Haruba Aoi, is a free-spirited and artistic teenager, often seen as the emotional core of the family. The youngest, Haruba Yui, is a precocious and curious child whose innocence and questions often drive the narrative forward.

The story unfolds through a series of episodic vignettes that explore the everyday lives of the Haruba siblings, their interactions with each other, and their relationships with friends, classmates, and neighbors. Each episode delves into themes of family bonds, personal growth, and the challenges of navigating adolescence. The siblings' distinct personalities and perspectives create a dynamic interplay, with Kei’s practicality often clashing with Aoi’s idealism, while Yui’s inquisitive nature bridges the gap between them.

Key plot points include Kei’s struggles with balancing his responsibilities as the eldest sibling with his own aspirations, Aoi’s journey to find her artistic voice amidst societal expectations, and Yui’s gradual understanding of the complexities of the world around her. The series also introduces supporting characters, such as classmates and teachers, who influence the siblings’ development and provide additional layers to the narrative. Through its slice-of-life approach, "Haruba-Ke no Sanninme" portrays the subtle yet profound moments that define family life, capturing both the humor and poignancy of growing up and growing together.