Youtarou, a high school student raised as an only child, abruptly finds himself thrust into a sprawling family after his grandmother’s death, uncovering a biological mother and 19 sisters spanning infancy to adulthood. His solitary existence shifts to a cacophony of sisterly interactions, requiring him to navigate a whirlwind of sibling dynamics. Older sisters react unpredictably—some embrace him as a newfound equal or pampered companion, while others bristle at his arrival, misreading his accidental mishaps as deliberate disruptions. Through it all, he maintains a patient, good-natured composure, slowly carving a place for himself by engaging earnestly in family life. Relationships with individual sisters unfold uniquely: Miharu, the eldest, subjects him to relentless teasing, while Haruka adopts respectful honorifics, her admiration growing steadily. Tsurara’s initial hostility, marked by dismissive labels like "manservant," thaws into a tsundere blend of sharpness and care after he aids her. Younger siblings like Mari and Watayuki shower him with open affection, the latter clinging to him with earnest "onii-chan" endearments. As the family’s reluctant mediator, Youtarou balances brotherly duties with the chaos of a female-dominated household, weathering absurd misunderstandings and accidental antics. His journey hinges on adapting to this abrupt identity shift—bridging age divides, mending fractured bonds, and reconciling his past solitude with the vibrant, overwhelming reality of an unconventional familial web.