Kazuya Kagami, a high school student, relocated to his school's dormitory after his parents sold their family home and moved abroad for business. With no dedicated boys' housing available, he became the sole male resident in the girls' dormitory among twenty female classmates. This arrangement placed him in a significant gender imbalance at an institution recently transitioned from an all-girls environment.

A pivotal moment unfolded when Kazuya secretly observed female classmates during a mandatory physical examination in the school's medical room. Witnessing their nudity, particularly their breasts, triggered intense arousal and became the catalyst for his motivations. Inspired by his cousin Tatsuya—who impregnated an entire class to form a loyal harem for broader world conquest—Kazuya vowed to achieve a parallel feat. He committed to acquiring his classmates' bodies and breasts through impregnation.

Kazuya systematically pursued each classmate, with most proving receptive to his advances. Exceptions included resistant individuals like the temperamental Akane and aloof Meiri, though these remained outliers. His tactics included daily breast massages for childhood friend Asuka Tooyama, increasing her bust size from 108cm to 114cm. Asuka, whom Kazuya had saved from drowning years earlier, viewed him as her lifesaver and aspired to marry him. Her devotion, exemplified by a room filled with stuffed animals, reflected the childlike dependency he cultivated among his conquests.

Leveraging the dormitory environment and his methods, Kazuya rapidly pursued every classmate to establish his own impregnated harem mirroring his cousin's achievement. His actions stemmed directly from the triggering observation and familial example, driving his focused objective throughout the narrative.