"Isekai Shōkan wa Nidome Desu" centers on Setsu Toujou, a man who was once summoned to a fantasy world as a hero but was betrayed and killed by his companions. After his death, he reincarnates back into his original world, retaining all his memories and abilities from the other world. Years later, he is unexpectedly summoned again to the same fantasy world, but this time, he is no longer the naive hero he once was. Armed with his past experiences and immense power, Setsu seeks revenge against those who wronged him while navigating the complexities of the world he thought he had left behind.

The story explores themes of betrayal, vengeance, and redemption as Setsu confronts his former allies, who have since become powerful figures in the fantasy world. His journey is marked by battles against formidable enemies, alliances with new companions, and the gradual unraveling of deeper conspiracies within the world. Key characters include Yuuhi, a young girl who becomes Setsu's companion and holds a mysterious connection to his past, and the former companions who betrayed him, each with their own motivations and secrets. The narrative balances action-packed sequences with moments of introspection, as Setsu grapples with his desire for revenge and the lingering sense of duty he feels toward the world he once tried to save.