Casshern Sins tells the story of a world where robots subjugated humanity after becoming self-aware. Their leader, Braiking Boss, ruled over the world with an iron fist. One day, a mysterious girl named Luna was summoned by the people in order to bring the salvation of mankind. Fearing her as a potential threat, Braiking Boss sent three of his most powerful cyborg warriors – Casshern, Dio, and Leda – to dispose of Luna. Casshern, the strongest warrior, managed to track down and kill Luna. However, this triggered a cataclysmic event which set into motion the end of the world. Hundreds of years later, the world’s atmosphere is filled with poison, and, due to the inability of most remaining humans to reproduce, as well as the constant threat posed by the robots, humanity is on the brink of extinction.
Robots fare little better, fearing death as much as humans do: the poisonous environment quickly causes their mechanical bodies to rust and corrode, forcing them to regularly replace their damaged parts, if spare parts in good condition can even be found. In this wretched time and place, Casshern, who had disappeared following the assassination of Luna, returns with no memory of who he is or what he had done