High school student Itsuki Amatsuki was summoned to a different world and fought as a hero to save the country. Despite his efforts to save the world, he was betrayed and killed by his trusted companions. However, when he awoke, he found himself summoned to a future world, this time as a hero to defeat the demon king that his past self had failed to vanquish. He realizes that among those ordering him to fight as a hero are the very same companions who betrayed him. Driven by a desire for revenge, Itsuki teams up with a young former demon king who shares his desire for vengeance, and together they set out to exact their revenge.