The plot follows a young man, Keita, who enters into a "contract" with a young Tera guardian, Kuro. The two fight evil and try to restore the balance of tera to the world. The series explores the dopple liner system, a system stating that there are three of everyone in the world, two 'subs' and one 'root'. If two of the three meet, the two will die and the remaining will become the 'root' and absorb all the remaining fate left over from the others.
Rin Asogi, which has a detective office in Shinjuku along with his assistant Mimi, meets Koki Maeno while he was looking for a stray cat. The boy says he has his own memories distorted, as if he really belonged. Proceeding with Maeno investigation is reconnected to Aoyama Pharmaceutical, where the guy turns out to be merely a clone developed in the laboratory. Although at first seems to have chosen the road of suicide, is the following day to the study of Rin to ask to become his partner.