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Keita is living any junior high school boy's dream. He has two older twin sisters, who compete for his romantic love in many ecchi ways, like good morning tongue kisses and abundant panty flashing. Their parents openly support the sisters' feelings and advances, because they are step-siblings and not related by blood. In fact they even have a bet going as to which sister Keita will choose. And since the sisters and Keita attend the same high school, there are bound to be numerous misunderstandings.

The story begins with Suminoe Keita, a male third-year junior-high school student studying for his high school entrance exams. He is living in a home with his older twin stepsisters, Ako and Riko, who kiss, flash, and display their love for him in public. They help him prepare for his exams. Although they share no blood-relation to him, Keita begins to become romantically attracted to his two stepsisters and initially dislikes himself for it. Seeing this attraction, his two parents encourage him to eventually get married to one of them.