Based on the erotic game by Marine.

On March 22, Saturday's summer holiday Shun was told to go back to his Nee-chan home to do something. Perhaps Shun thought he would be made into a toy By Nee-chan. And by the time Shun comes home, Nee-chan tells Shun because they get an unknown disease that's Idiopathic Estrus Syndrome, that causes his sister to feel the heat in his body that is out of control and the only way to cure the disease. They have to Regular intake of male sperm. And the romance relationship between Nee-chan and Shun begins ...

(Source: Nekopoi and Has Been Translated and Edited)