Koichi was Kokoro by the sounds of his mother Yuri's moans coming from his parents' bedroom. He was aware that his parents were planning to have a child. From the son's point of view, this might be something to be celebrated. However, when he discovered that Yuri had a lewd side to her that he had never been exposed to, uncontrollable emotions welled up within Koichi. These were jealousy towards his father. And a possessive desire to have Yuri all for himself. When these emotions reached their limit, Koichi's twisted love for Yuri crossed the forbidden line.