Based on the erotic game by Caramel Box.

Ryouhei was suddenly be called back to his hometown in the countryside. There, he meets Karen's mascot-girl from the inn's house. As she stands entranced by the sight of her bewitching proportions, Nagisa, the younger brother of Ryouhei's ex-boyfriend and the man who introduced her to Karen, looks on uneasily.

(Source: Nekopoi and Has Been Translated)