In a world where the powerful robot leader known as Braiking Boss rule over humanity with an iron fist, a mysterious girl named Luna is summoned by the people for salvation. To retaliate against this potential threat, Braiking sends 3 of his most powerful cyborgs, Casshern, Dio and Reda to dispose of her. Casshern manages to kill Luna, plunging the world into a slow descent of annhilation. Hundreds of years later, humanity is almost extinct. Robots constantly replace their body parts, fearing the corrosion of rust: their only form of death. In this wretched time and place, Casshern returns after a lengthy disappearance since his murder of Luna. Having lost his memory, Casshern now defends the humans from the robots, and find out his true identity and purpose.