Based on the erotic game by Bishop.<br>
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Ryuya has an older sister, Miya.  He lives a peaceful school life with his friends.  But one day, he and his friends accidentally drink a medicine juice.  Once you drink it, you'll become sexually sensitive and get excited.  The breasts of the girls who drink it will get bigger and they will need to have sex to cool down.  Also, to make a vaccine, they will need to get pregnant.  Ryuya starts having sex with the girls to cool them down and make a vaccine.<br>
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(Source: ErogeShop)