One day, the little brother is possessed by a sex demon.
Our hero, Shinonome Kei, is possessed by a sex demon named Ink. A depraved kind of illness befalls him! If he is to be saved, he must have sex until he ejaculates. Ink's target is Kei's elder sister, Otoka. She coerces Otoka to seduce and have sex with her little brother in order to save his life.

[From DLsite English]