The brilliant Dr. Azuma designed numerous robots to assist humanity. But during a thunderstorm, lightning struck his laboratory, corrupting the programming of one of his creations, BK-1, who renames himself Braiking Boss and proclaims that rather than serving humans, humans will now serve him! He assembles other robots into a massive army determined to enslave the human race.
Tetsuya, Dr. Azuma's son, begs his father to transform him into an indestructible android so he can have the strength to defeat the menace his father unintentionally created. Soon after, Braiking Boss kidnaps Dr. Azuma and his wife. Tetsuya, now calling himself Casshern, embarks on a mission to reverse the war's outcome and rescue his parents, supported by his girlfriend Luna and his robotic dog Friender.