Pink no Rou Maa ~Rin to Mai no Midare Kusuri~

In 2059 AD human population increases greatly, and Yuki Rin in her sexy costume is dispatched to the past to learn the reason. While working at a coffee shop for men she finds girl Mai who is drugged and forced to participate in creation of adult video. Rin opposes this underground lab members, but gets caught herself. The culprit manages to stick his poisonous fangs into Mai's body before escaping to the future. With the poison spread the future is doomed again, so Rin's battle for better tomorrow begins.

[From vndbreview]