The female protagonist, Mei, has a sexual relationship with her childhood friends, the two brothers Aki and Yuuki. They promised to always have threesome sex together using contraception. Although they have been intimate with each other, they have never been able to tell each other their true feelings.

Living such a life, the three of them can't suppress their lust for each other.
One day, Mei is barebacked by Yuuki.
Aki finds out and...

Despite her pleas for forgiveness, Mei has been repeatedly pounded by Aki. She's made to enjoy being penetrated and ejaculated into.
Yuuki, who wants to show Mei his dominance, puts the rotor on her own genital and pushes against her cervix, making Mei come again and again.

And the three of them get the news. Mei is pregnant with Aki and Yuuki's child.
As she sucks on both of their penises at the same time and learns new pleasures, the twisted relationship between the three of them becomes irreversible...

Official Translations:
English, S.Chinese, T.Chinese, Korean