Wanting to have sex, Hajime goes to his girlfriend's house for the first time, and even though they've been dating for a long time, his girlfriend is still too serious and won't let him have sex with her. He finds an unfathomable liking for her younger sister Mao, who is the complete opposite: open when it comes to sex, full of flaws, and a bold personality... His girlfriend still won't let him have sex with her, and her younger sister is fair-skinned, big-breasted, and open when it comes to sex... And what's more, his girlfriend's sister, who is in the mood for sex, whispers dirty talk and he easily falls for her, becoming immersed in an affair. This is the story of a slutty guy.