The empire's secret policeman, Kotomine Kirei, has been deeply in love with his mentor, Tokiomi Tosaka, since he was a boy. However, a few years later, Tokiomi encountered an accident, and according to his last wish, part of his body was transformed into an automaton-TS-06K.
Faced with the emotionally indifferent Automata that inherited all of Tokiomi's memories and knowledge, Kirei still couldn't stop feeling those secret feelings. But one day, "it" was stolen by Emiya Kiritsugu, a heretic who opposed the imperial rule...