Having lost their parents at a young age, older sister Kyoka has worked hard to raise her younger brother. This summer, she finally becomes engaged, and her fiancé starts visiting the Ishiguro household daily. Amid this joyful atmosphere, the brother begins to feel an inexplicable tightness in his chest. For him, who has cherished Kyoka like a mother, she has always been more than just a sister—she's been his emotional anchor. Yet, over time, his feelings for her begin to shift into something beyond familial bonds.