Yūta Segawa, orphaned in childhood, was raised by his older sister Yuri. Resentment toward her marriage to Shingo Takanashi, a widower with two daughters, led Yūta to live independently for five years before starting university. As a Tama University literature freshman, he reluctantly agreed to briefly care for his three nieces—Sora, Miu, and Hina—during their parents' trip. When Yuri and Shingo's plane disappeared, Yūta defied extended family plans to separate the sisters, assuming guardianship despite having no resources.

Initial struggles included severe financial strain, cramped apartment living, and balancing childcare with academics. His relationships with each niece developed uniquely: toddler Hina, his biological niece, showed immediate affection with her distinct speech ("Oi-tan"); Miu tested boundaries through her interest in older men; Sora exhibited tsundere behavior stemming from a longstanding crush dating to their first meeting years earlier. Externally, his involvement with the Street Observation Research Society included friendship with Raika Oda, complicated by unexpressed romantic feelings and his new family duties.

Yūta demonstrated reliability by managing crises, protecting the girls from predatory individuals, and negotiating with skeptical relatives. A significant turning point came when the Takanashi family acknowledged his commitment, increasing financial support and granting shared ownership of Yuri's house, enabling relocation and stability. Over subsequent years, Hina entered elementary school with improved speech; Miu pursued junior high idol activities; Sora advanced to high school while leading her choir club.

Romantic resolutions diverged across adaptations: the anime concluded with a symbolic daycare event proposal to Raika, while light novels depicted engagement and marriage to adult Sora, including her pregnancy. Additional traits included accidental pervert incidents, a disclosed cat-girl fetish in manga chapters, and protective "papa wolf" instincts against perceived threats. The narrative consistently emphasized themes of chosen family, responsibility, and gradual maturity.