After his sister suggested it, Shunsuke Ayakawa visits his home during summer vacation. There awaits him his older sisters Asaka, Tsukino and Yuki. The three of them are very happy to have him around, because they are all in need of him. It seems they all got sick with a life threatening disease called “Sudden Sexual Arousal Syndrome”, which forces them to intake a adequate amount of a man’s semen on a regular basis. Not to be forced to have sex with untrustworthy strangers and beastly men, they seek help from the cute little brother to give them their “medication”. Based on the adult PC game by Marine (brand of Marigold).