女装で孕ませてっ外伝～過去は夕闇色の少女と共に～（Josou de Haramasete Gaiden ~Kako wa Yuuyami Iro no Shoujo to Tomo ni~）

The main character, Yori, was approached by her cousin, Yuu, in an attempt to have a child. Yori is trying to have a child with her cousin, and is approaching him in various ways.

Then, a girl transfers to the girls' school. Her name is Shigure. She is somewhat mysterious. She approaches Yori and Yuu and their friends with a mysterious look on her face, trying to keep them from getting together.

Shigure takes Yuu and Yori out on dates to seduce them, and they get involved in various ways.

At the same time, a strange disease begins to spread in the town, causing people to collapse one after another. people fall over one after another in the city. It seems that all the people who have fallen ill have their energy drained out of them

They all seem to be drained of their energy and fall asleep. Yuu's mother, Aki, is also afflicted with such a disease.

Why does Shigure want to prevent Yuu from getting together with Yori?

Is there a connection between Shgure and the strange disease?

What will happen to Yuu and Yori?

[From official site]