People won't continue doing something if they believe it's wrong. However, if they delude themselves into thinking it's a good deed, then...
Takamura "Nono" Nodoka is a friendless, lonely girl from a single-mother household. One day, she becomes close with Ichijou Masamune, a single man in his 30s who lives alone in a palatial mansion famous in the neighborhood.
Nono thought, "I don't want to be all alone during this year's summer holidays. I want to spend them with him!" and barged into Masamune's home. However, in exchange for letting her stay at his place, Masamune demands Nono to show him her butthole.
It turns out that he's a super-perverted anal sex maniac!
What will become of Nono's chastity?!
In this ADV, a lonely, pure(?!) and innocent(?!) girl spends her summer holidays receiving loads of anal sex!!
(...Is just what it seems like but perhaps it's the older man in his thirties who's really losing himself in this story.)
[Translation of DL Site's story summary.]