On a day when Kenichi's wife, Risa, was staying at her parents' house close to giving birth, Kenichi was enjoying the freedom of living alone. Suddenly, Risa's older sister, Yumiko, showed up unannounced. Yumiko's visit wasn't just out of concern—it stirred something deep within Kenichi. Though he had grown accustomed to life with Risa, he found himself increasingly drawn to Yumiko. He realized feelings for another woman were awakening inside him, but he had no confidence he could suppress them. As if sensing his inner turmoil, Yumiko began whispering seductive words, shaking his resolve. In that moment, their relationship took a bold, unexpected turn.