Sakuramiya Saya and Hinata are sisters, members of the archery club, and also a couple. After their parents remarried, they became sisters, and then had a relationship after that.

Tachikawa of the photography club found out about their relationship and blackmailed them. He starts with the younger one, Hinata. He takes a video of him breaking her hymen as he forces her to have sex with him. Worried about Hinata, Saya goes to the clubroom to check on her, only to be drawn in. They end up both getting raped and cummed on.

And with that starts their days of being played by Tachikawa and raped every single day. His demands eventually escalate, and eventually they are raped by not just Tachikawa but many others...

(Source: hanime.tv)