His younger sister, Riri-chan, is cuter than his girlfriend, more innocent, and his type. However, ever since they met, there's something that's been bothering him... the summer fashion that girls these days wear expose a lot of skin. What's more, when she's relaxing at home, her pussy line is clearly visible under her skirt. What's more, when she bends over without a bra, he can see her breasts, and he can't help but look at this lucky pervert. Even though he knows it's wrong, his reason is blown away by her defenseless temptation, and before he knows it, he's squeezing her breasts and pistoning away. Even though his girlfriend is right next to him... he is consumed by the sinful pleasure and overheats as he thrusts into her until he gets heatstroke. A memory of a summer when their sexual desires couldn't be contained and they devoured each other wildly...