Keisuke Tsukikawa, the main character whose sister-in-law's personality has split and has become two younger sisters She comes to the beach with her sisters Saki Tsukikawa and Saki (a split of Saki) and her classmate Riri Manabe (succubus). Saki, who first learns that Kasuke and her blood are not connected, considers Saki's feelings that were sealed. Saki and Keisuke are alone in the shade of a rock. So Saki asks her to do her actions when Saki and her personality have changed, and Keisuke accepts her Saki. That night, Saki, Saki, and Riri attack Keisuke who takes an open-air bath. Succubus, who originally have strong sexual desire, and Saki, who is influenced by it and has a nasty nature, urges her favorite Kasuke to seed her. Keisuke, who is at a loss if he cannot love the three in the same way, gives one answer to his smiling sisters who have not changed since childhood.