After twenty years of failure and despair, Noboru wanders into a deep forest only to stumble upon a hidden village of horned demon women. Following a night of hedonistic indulgence with the inhabitants and their Queen, he is informed by a guide named Kirika that his new purpose in life is to impregnate the village's population of 10,000 girls.
Noboru discovers he is in a secluded village of Oni girls, where he is tasked with the monumental goal of impregnating ten thousand residents. After learning about the biological link between the girls' breasts and their demon nature, Noboru is thrust into a hedonistic welcome ceremony known as the "Breast's Arc." Overwhelmed by the girls' intense desires, he begins his mission to father a new generation of demons.
Exhausted from his duties with his harem, Noboru discovers a revitalizing open-air milk bath used by the Oni Princesses. With his energy restored by the magical waters, he engages in passionate encounters with Village Chief Dakki and Secretary Kirika, both of whom are desperate to be impregnated.
Noboru continues his carnal encounters with Kirika and Dakki before meeting Shirohime, Dakki's daughter. Shirohime warns him that men usually perish in their town due to energy depletion, but Noboru's enthusiasm for the "breast paradise" leads her to offer him her virginity. Ultimately, the demon administration decides to keep Noboru alive so he can lead a permanent harem life among them.