The brilliant Dr. Azuma developed numerous robots designed to assist humanity. However, a lightning strike during a thunderstorm hit his laboratory, corrupting the programming of one robot, BK-1. This unit renamed himself Braiking Boss and declared that instead of serving mankind, humanity would now serve him. He assembled other robots into an overwhelming army dedicated to enslaving the human race. Tetsuya, Dr. Azuma's son, requested that his father transform him into an indestructible android, seeking the power to eliminate the threat his father had inadvertently created. Shortly afterward, Braiking Boss captured Dr. Azuma and his wife. Tetsuya, now renamed Casshern, set out to turn the tide of the war and rescue his parents, aided by his girlfriend, Luna, and his robot dog, Friender.