Based on the eroge (erotic game) by LiLiM DARKNESS.

Asato and Akane have been together for years, but their relationship remains platonic. Asato thought their happiness would last forever. However one day Kyousuke, Akane's childhool friend, suddenly came to them and begged to be allowed to stay with them for a short time. Because Akane wanted to help her friend they agreed, but neither of them suspected Kyousuke's true intentions...