Rimu, the younger sister, is shy and quiet even in front of her family, but she actually has a strong curiosity about sex and harbors feelings for her older brother Hajime. One day, Hajime was entrusted by his parents to stay home while they attended a relative's funeral service. When Rimu confessed her feelings to Hajime, he initially rejected her saying, 'We're siblings,' but when pressured with 'It's just for fun, even if it's only a physical relationship,' he crossed the line. After that, the two of them went wild, indulging in sex for the two days their parents were away.
