Built underground at the Gyokusen caves in Okinawa by the aliens of the Third Planet of the Black Hole, Mechagodzilla was a massive war machine. After its Fake Godzilla disguise was compromised, the machine burnt it to reveal its true identity. Fashioned from Space Titanium, ten time stronger than steel, the robot was ready for conquest. Its first goal was to remove the creature it was based on: Godzilla.
With gleaming ember eyes, Mechagodzilla wasted no time in proving its power. Following its commands, the mechanical titan immediately unleashed waves of finger rockets. Caught in the explosion, Godzilla was thrown to the ground. Seizing the opportunity, the mechanical doppelganger advanced. Leaning in, the robot fired a blast of its Space Beams from its eyes. Godzilla suddenly sat up, though, narrowly avoiding the beams. As parts of the refinery exploded from the blast, the nuclear leviathan stood again to fight. Silent rage burned as Godzilla unleashed his Atomic Ray. Answering Godzilla's sea blue nuclear breath, the machine unleashed its Space Beams again. Both godly forces struck, pushing back and forth. However, only one outcome was possible. The combined forces exploded with ear shattering force, knocking both competitors back. Godzilla fell into the bay, leaving only a churning swirl of blood to mark its presence. Mechagodzilla, meanwhile, suffered major damage to its head control unit. This forced the aliens to withdraw and repair their war machine.
Recalling it back to their Okinawa base, repairs on the robot became time consuming. Concerns mounted that certain Black Hole aliens would realize the mistakes surrounding the Mechagodzilla project. However, the project leader, Kuronuma, had a plan. To hasten the repairs, they forced a human scientist named Hideto Miyajima, whom who had some knowledge of the alien technology, to fix it. The gamble paid off for the aliens, as the machine was repaired and ready for combat.
Launching from their Okinawa base, Mechagodzilla was ready to strike once more. This time, though, humanity was prepared. Awaking the legendary protector god, King Caesar, the prophecy, carved in the Azumi Royal Family's cave that told of an epic war, was becoming a reality. The two monsters fought heatedly, but not even the giant golem-like kaiju could overcome the metal warrior. Giving ground, it looked as if the god-like creature was to be defeated. That is until Godzilla broke through the bay's surface. Marching inland, Godzilla seethed with fury.
This turned the conflict into a two-on-one, as Godzilla and King Caesar united against a common foe. However, superior numbers did little to turn the tide. Mechagodzilla proved powerful enough to keep its two attackers on the ropes. It wasn't until Godzilla, using an electric charge he absorbed earlier, turned himself into a magnetic force did the tides turn. Using this new power, the King of the Monsters drew the metal monster toward him. Held in place, King Caesar slammed into the robot. The fierce assault overcome the omnipotent strength of the machine. Looking to finish the battle, Godzilla used his brute strength to twist the machine's head. The nuclear monster then ripped off Mechagodzilla's head, ending its reign. Defeated, Mechagodzilla exploded into pieces of shrapnel, ending the alien's plans for the time being.