During an archeological expedition on the Aegean Sea, two world renowned scientists, Dr. Hell and Dr. Kabuto, discovered the secret weapons of the Mikenese Empire, giant robots, which the empire used to protect their many treasures and drive invaders away from their island.
Whereas everyone else thought that these findings were of great positive value to humanity Dr. Hell saw them as a tool that he could use to take over the world. Immediately he ordered the Mikenese robots to attack the rest of the expeditionary group so that no one else knew of his plans. However, Dr. Kabuto managed to escape and go back to Japan, where he began working on the only weapon that would be able to stand up to Dr. Hell's plans for conquest, Mazinger-Z.
Dr. Kabuto's robot, Mazinger-Z was the result of two of the Doctor's secret projects, a new metal alloy known as super alloy Z (composed of a new element discovered on the stratum of Mt. Fuji known as Japanium) and a new energy source (Photo-Atomic energy). However before he could tell anyone about Mazinger-Z Dr. Hell's minions, lead by Baron Ashura, caught up to Kabuto and were quick to bomb his home/laboratory. Thankfully Dr. Kabuto's grandchildren, Kouji and Shiro arrived at his demolished home before he passed away, the dying man was able to tell them about Mazinger-Z and Dr. Hell's nefarious plans.
Angered by his grandfather's death Kouji set out to learn how to use Mazinger-Z and even though at first he didn't know how to control the machine. He eventually mastered the robot's controls and with the help of the Photo-Atomic Research Institute began his long battle against Dr. Hell's robot army.