Dans un futur lointain, les cyborgs et humains luttent pour survivre suite à la guerre qui a détruit le monde. C'est dans ce monde que se réveille un cyborg, Casshern, qui a perdu toute notion de son passé. Seul quelques flashs de sa vie antérieure lui reviennent. Il se retrouve alors à défendre les hommes contre de puissants robots qui règnent d'une main de fer sur le reste de l'humanité puisqu'il est le seul à avoir une puissance suffisante pour les contrer. Il n'aura qu'une seul hantise : découvrir qui il est et que cache son passé enfouit au plus profond de sa mémoire cybernétique ?

Tetsuya Azuma, also known as Casshern, is an android with a human consciousness neoroider. Tetsuya turned himself into an android for the purpose of hunting down and destroying robots that have taken over the world. His biological father, Dr. Kotaro Azuma, was the inventor of the automatons that were originally intended to serve humankind. However, the first android, BK-1, was struck by lightning, and went out of control. With its great strength, it escaped from the castle despite attempts to stop it. After some time, it renamed itself as Buraiking Boss (often mistranslated in English as "Black King Boss"; derived from ?? or burai, meaning rogue or brute), and was able to build a robot army against mankind; the robots mutinied en masse when their cold logic concluded that the good of the Earth Ecosystem required the destruction of the human race. Casshern and his robotic dog, Friender, join forces with a beautiful girl named Luna Kozuki to battle the robots led by the Buraiking Boss. Casshern has great strength and agility, but he is not armed, except for a pair of odd pistols, used more like rockets than weapons. Friender is capable of transforming itself into a tank or a jet aircraft and actively helps Casshern to fight the robot army. While the robots are huge and robust machines, he is usually able to destroy them with his bare hands, dispatching a great number in any battle. Casshern has also some weakpoints: his body must be re-charged (with solar energy), and cannot sustain very long battles without risking low battery power. Luna was totally harmless, until her father was able to build an electromagnetic pistol, easily capable of destroying the robots. Almost all the robot machines have an antenna on top of their heads, ripping it off usually causes them to blow up, so they are relatively vulnerable. (Wikipedia)

Depuis un certain temps, le mystérieux "Z" voyage de monde en monde afin de tous les détruire. Il ne souhaite en conserver qu'un pour assouvir ses projets. Mais alors qu'il combat Gatchaman, il perd la source de son pouvoir, qui atterrit entre les mains de la jeune Emi Kaidô. Cette dernière va dès lors être protégée par des héros venus d'autres mondes : Polymar, Gatchaman, Tekkaman et Casshern.