By some sort of fate, exorcist xiao-Yu is forced to "live under the same roof" with the succubus named Obsidian, who killed her adoptive father. Yet she discovers that the succubus seems to have lost her memories! Unable to accept the 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... Although they seem to have a love-hate relationship, they both know that something dangerous is stirring beneath the depths. Something related to why her adoptive father was killed, why Obsidian's memory was sealed away, and...