A high school student, Amagi Iori, was summoned to another world and fought as a hero to save the country. He tried to save the world, but was killed by the comrades he trusted. Though thought dead, Iori awakens in the future, summoned as a hero to defeat the Demon Lord he once failed to vanquish. He then realizes that among those ordering him to fight as a hero are his former comrades. "...I'll make you regret betraying me." Driven by hatred, a former demon lord girl who also seeks revenge appears before Iori. The hero, having abandoned his ideals and returned to this world, sets out for revenge with the former demon lord.