Kotone is the pure and innocent sister from ● High School, sharing a close bond with her older brother since childhood. She deeply adores her brother, who has always doted on her. As she enters puberty, she begins reflecting on her emotions and finds herself excited by kissing scenes on TV. One day, she nervously asks her brother, "How exactly do you kiss?" Her brother, a kind and caring university student who cherishes his sister, is initially surprised. But sensing her genuine curiosity, he gently shows her how to kiss. However, the moment their lips meet, he starts seeing her not just as a sister, but as a woman. Though he knows incest is wrong, what truly lies in his heart?