Yuina loves her non-biological big brother. One night as she thinks about her brother, she suddenly finds herself in a new world. She gets harassed by a man, whereupon a succubus appears in front of her and saves her. Succu, the succubus, explains to Yuina that she can return to her old world via a portal. In order to create a large enough portal she has to become a succubus herself and in order to do that she has to seduce enough men and so the adventure of Yuina and Succu begins.