In the Kansai region, there lived a 30-year-old woman named Mami. She had just moved back to her hometown after years of living in the city. Mami had decided to leave her corporate job to care for her 13-year-old niece, Yui, whose parents had passed away in an accident. Yui was a bright and energetic junior high school student, but she had been feeling lost and alone since the tragedy.