My sister-in-law Yui has always been by my side since we were kids, calling me "big brother, big brother" and playing with me. Lately, though, she's started showing increasingly erotic behavior—deliberately exposing her thighs and ass, acting provocatively. Though I try to maintain a sibling relationship, I can't help but be drawn to her. One day, I never expected her to catch me masturbating. With our parents away for two days, we're left alone together, and my desire quickly spirals out of control. In this intimate time away from our ordinary lives, our relationship begins to shift in unexpected ways.