During summer vacation, Shota-kun goes to the beach with his cousin, older sister Sara. Entranced by her new bikini hugging her plump, shapely buttocks, his heart races as each day unfolds. Every time they play in the sea, his desire to touch her round hips grows stronger. Back at the hotel, they enjoy mixed bathing together. Pretending to wash off sand, Shota-kun sneaks touches on her backside, and teases her during oil massage sessions—each day turning into a steamy, erotic adventure. Even while his older sister sleeps, Shota-kun innocently reaches out to caress her lush curves, weaving a sweet yet lustful love story.