Shota, a reclusive boy, had only opened up to his sole friend, Kisei. One day, Kisei starts visiting Shota's house with his older sister Rina, and Shota becomes increasingly fascinated by Rina's big tits. He gradually begins teasing her in subtle ways. At first, Rina resists, but after realizing "if it helps Shota-kun become more lively," she slowly starts accepting his naughty pranks. Their relationship evolves from pure friendship into something deeper, transforming into a new kind of bond.