In line with the eroge (erotic game) by LiLiM DARKNESS.

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