Rimu, the younger sister, is shy and quiet even in front of her family, but she has a strong sexual Kokoro and is in love with her older brother, Hajime. One day, Hajime is left in charge of watching the house while his parents go to a relative's memorial service. When Rimu confesses her feelings to him, Hajime initially brushes her off, saying, "We're brothers, aren't we?" But when Rimu presses him, saying, "It's okay to just play, I don't mind if we're just physically together," he crosses the line. After that, the two of them seem to let their inhibitions go and have sex as their carnal desires dictate for two days while their parents are away.