The monsters of this world exist everywhere. They hide in plain sight, and attack when your blood is warmest and senses are dimmed. These monsters are called Oni. Among the humans, are a certain group with mixed blood, three quarters human, one quarter monster. They are destined to fight off the dangerous creatures of this world, however, those dangerous creatures are also out to exterminate them. The mixed blood attracts the monsters, as if the humans are asking for constant attack on them and the ones closest to them.

Along came a young mixed blood, Yin. She is one of the last of her kind, but she is faithful to her kind's ancient duties- To put their lives before the humans and to kill any monster in their sight. Yin has always been unnoticed and figures that her job comes with her cursed blood. Not once has she fully doubted her natural instincts to kill, but also to be invisible to humans... Until Riku came. He's the only human she's willing to die for on a personal level.