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.