A colossal corpse was uncovered in Japan, and it appeared to be an enormous, avian abomination. The monster was relocated to a research lab in Tokyo, and the true identity of this relic soon became known to three humans. Two diminutive fairies, called the Shobijin, revealed to the humans that this was a monster that had battled with the guardian Mothra. Gigan, as the sinister cyborg was known, had fought against the peaceful protector over 10,000 years ago, and unless its dormant body was returned to its tomb, dire consequences would befall the world. The humans didn't heed the warning, and careless research of the beast continued...

Meanwhile, the deceitful extra terrestrial race known as the Xiliens had taken responsibility for the recent defeat of several dangerous monsters around the world. Their true scheme was to use the monster invasion as their first phase of domination, and when this fact came to light, the Xiliens attacked the Earth once more! During this second assault, the cybernetic organism, a weapon of the alien race, was revived and ordered to attack Tokyo; however, when Gotengo took flight to Antarctica to free Godzilla, the Xiliens ordered Gigan to follow and destroy the aerial vessel.

When the Gotengo arrived at the very bottom of the Earth, Gigan began to unleash its full fury on the battleship. The Gotengo was bombarded with attacks and was finally forced to the ground. It did, however, manage to fire its missiles at the base that held Godzilla. Right before Gigan was about to finish the Gotengo, an atomic ray blasted into its flesh, sending the contemptible creature to the ground. The king of the monsters had risen from his forty-year hibernation, and Gigan retaliated with its powerful cluster beam. As the reprehensible, robotic terror wrapped its cables around the nuclear menace and began to real him in, Gigan's abdominal buzz saw began to spin violently, and it appeared as though Godzilla would soon meet a bloody fate. A single atomic ray decapitated Gigan, however, and the hateful horror fell to the ground. Godzilla roared in victory and began to follow Gotengo back to Tokyo, unaware that the creature lying behind him was not completely defeated...

Following its decapitation in Antarctica, Gigan was repaired and its arsenal was enhanced. It was time for the Xiliens to send this cybernetic leviathan back into battle. Luckily for humankind, Gigan's ancient enemy, Mothra, arrived to assist Godzilla in his fight against Monster X. However, upon the sudden appearance of the newly modified Gigan, Mothra switched her attention to this new foe. The majestic monster soon began to lose the upper hand, and it began to retreat. Gigan followed and used its chainsaw-like appendage to lop off the bottom of Mothra's wing, sending her plummeting to the ground, unconscious.

Convinced that its opponent was down for the count, Gigan raced over to assist Monster X in its fight against Godzilla. Monster X held the King of the Monsters, as the sadistic cyborg began to use its chainsaws to slice into Godzilla's pectoral region. The tables were turned, however, when Godzilla managed to lure Gigan into cutting Monster X. Suddenly, Mothra reentered the fray and slammed both of the evil incarnates to the ground. Gigan and Mothra fought furiously once more! This time, Gigan unleashed its spinning disks, but the agile arthropod dodged the deadly blades. The disks began to boomerang back toward Gigan, as Mothra was struck by her enemy's cluster beam. She erupted into a glowing blaze of crimson fire, as Gigan's razor disks struck its neck. For a moment, it appeared to have done no damage whatsoever. As the cyborg turned to finish the job, its head toppled to the ground. A conflagrant Mothra charged Gigan and caused its entire body to erupt into flames. Gigan burst into pieces, and this frightful foe was finally finished.