In the throes of a heated argument, Riho, the elder sister, finds herself face-to-face with her younger brother in their cramped, cluttered room. As they bicker, Riho's eyes flick down to catch a glimpse of her brother's growing bulge, igniting an unexpected spark within her. She bites her lip, her mind racing with forbidden thoughts as she imagines the length and thickness of his member. Their heated exchange turns into a battle of wills, each pushing the other's buttons, until Riho can't help but blurt out her taboo desire.