Yuina loves her non-biological older brother. One night, while thinking about him, she suddenly finds herself transported to a new world. After being harassed by a man, a succubus appears and saves her. This succubus, named Succu, explains to Yuina that she can return to her original world through a portal. To create a portal large enough, Yuina must become a succubus herself, which requires her to seduce enough men. Thus, the adventure of Yuina and Succu begins.