Oyasumi Sex alternates between the views of the siblings Akira and Yui, who have a normal and peaceful relationship with each other. However, when their parents divorce, the young Yui is devastated and clings to her brother all the more, who has decided to take good care of her. As the years pass and Yui gets older and bigger, she starts to develop certain qualities that slowly lead to Akira developing lewd thoughts and making it more and more difficult for him to control his desires. Where there used to be care and love, now there is only carnal yearning.