"Satomi-chan" is a high school girl who is sweet and affectionate toward her older brother, cherishing every moment they spend together—so much that even small physical contact makes her heart race. One day, while playfully touching each other with her beloved older brother, she suddenly feels an itchy, tingling sensation between her legs. She shyly asks him to check what's wrong, and he gently touches her. To her shock, he discovers her panties are completely soaked. She's left speechless, realizing too late that her relationship with her brother has quietly turned into something far more intimate and erotic.