Kazuhiko's stepmother, Miku, who was born when his father remarried, was a woman closer in age to Kazuhiko than his father. Although he felt a slight repulsion towards her, he was Kokoro to her beauty. As the father-son relationship deepened, Kazuhiko happened to see his father and Miku having sex. It was a sight far too stimulating for a virgin. Kazuhiko lost control and begged Miku for sex... and from there, the forbidden relationship between the two began...