Sari Kosaka suddenly becomes his stepsister after their parents' unexpected remarriage. She appears to be a calm, beautiful girl with a big bust and gentle personality, but secretly has a mischievous, seductive side. During their first meeting, she shocks him by suddenly saying, "Let me do something nice for you to celebrate becoming siblings," then grabs his crotch, lets him grope her breasts, and gives him two full handjobs. From then on, they sneak around behind their parents' backs, escalating from blowjobs and titty fucks to full sex. Pressing her large breasts against him and clinging to him cutely, her sly charm becomes irresistible. The story captures how her alluring appeal gradually entangles both his heart and body.