Rui Kamishiro is a mad scientist whose hobby is creating living organisms that quickly die. One day he created a living organism that is very similar to a human teenager. He named it Tsukasa, but he calls it Tsu. Only when he is angry does he call it by its full name. Tsukasa can regenerate its wounds when it sleeps, but it has very weak immunity. Rui cares for him and has made his body dependent on him and devoted to him, although fear is at the root of their relationship.