By some sort of fate, exorcist Xiao Yu is forced to "cohabitate" with the succubus named Obsidian, who killed her adoptive father. Yet she discovers that the succubus seems to have lost her memories! Unaccepting of this fact, Xiao Yu decides to spy on Obsidian's every move, yet the latter doesn't seem bothered by the least. Instead, she starts to thirst after Xiao Yu openly and often teases her body to satiate her own hunger...