Yumino Rimu, the younger sister, appears shy and well-behaved in front of her family, but secretly harbors intense feelings for her older brother Hajime. One day, when their parents leave Hajime in charge of the house, Rimu confesses her emotions. At first, he resists, saying, "We're siblings," but when she pleads, "It's okay if it's just for fun, I just want a physical relationship," he finally gives in. Over the next two days while their parents are away, the two surrender completely to their passion. As the bond of brother and sister intertwines with their sexual desires, a new boundary is forged between them.