Performed by adults. Ai and he are not biological siblings; she became his older stepsister when their parents married while he was an adult student. After their parents died a year later, Ai took on the role of his guardian. As the two lived alone together, he fell in love with his only remaining family member and vowed to make her happy someday. Then his sleazy friend Shinji entered their lives, remarked on how sexy and beautiful Ai was, and began spending all his time at their home...