When Koji's wife returns to her hometown to give birth, her sister Mizuki offers to look after him. Koji, who was struggling to balance work and housework, gladly accepts, but Mizuki even offers to help him bathe, and he ends up taking a bath with her. His cock cannot help but react to Mizuki's beautiful naked body. Noticing this, Mizuki comes on to him forcefully, saying, "Did you get hard because of me? I'm so happy..."