After coming home with his girlfriend, the young man meets Akari, the plump and cute high school sister, and begins to have feelings for her due to her charming and irresistible charm. On a rainy night, as she forgot to bring a raincoat to go to extra classes, the younger sister had to stop by the older sister's house temporarily, but only the boyfriend was home as the older sister couldn't come back due to the heavy rain. Afraid of the thunder and lightning, as soon as she stepped into the house, she quickly knelt down and hugged his legs tightly because of the fierce thunder, revealing her alluring curves in the wet white high school uniform. Unable to control himself at the sight, he pushed her onto the sofa and began to satisfy his desires, even though at first the younger sister was still shy and whimpering in protest. The story didn't stop there, from that point on, she often felt a strong sexual desire, so she took advantage of the opportunity when her older sister went on a business trip for 3 days to continue the affair with him - her older sister's soon-to-be husband.