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 ...