Enter Kazuya, a boy who never had any memories before his adolescent life. He was summoned to the world of Edea where in the final battle against the Demon King, our hero entered the last room to find a welcome he had not been expecting. An unguarded, unarmed demon king who greeted him with manners and etiquette.
As if a respectful and well mannered final boss wasn't shocking enough, he was then introduced into an elaborate plot to exploit summoned heroes. Unfortunately, our naive hero chose to trust his companions. He underestimated human greed which led him to suffer alone in darkness for a time he himself does not know how long. 
When he suddenly woke up, he was surprised to see a younger version of himself. As an amnesiac, he had been thrown into an unknown part of his life. One thing never changed though, his blood-filled desire to get back at those who made him realize his fall from being a hero to a pawn.
Follow along as Kazuya finds out about his past and tries to prevent the tragedies he's experienced while discovering what caused them to change the tragic life he led into one that he could accept.