In Okinawa, a lone Azumi priestess receives a chilling vision of a city scorched by a colossal force. Masahiko Shimizu [Kazuya Aoyama] discovers space titanium while spelunking and brings it to Professor Hideto Miyajima [Akihiko Hirata] for study. An opening excavation led by Masahiko’s brother, Keisuke Shimizu [Masaaki Daimon], uncovers a chamber filled with ancient artifacts and a mural prophesying catastrophe: “When a black mountain appears above the clouds, a huge monster will arise and try to destroy the world; but when the red moon sets and the sun rises in the west, two monsters shall appear to save the people.” Keisuke teams up with archaeologist Saeko Kanagusuku [Reiko Tajima], who translates the prophecy and examines a statue bearing the likeness of Okinawa’s guardian monster King Caesar [Kinichi Kusumi]. Two men shadow them—one posing as a reporter, the other attempting to steal the statue—hinting at a danger far larger than they realize.
A black cloud thickens above the mountains and a Godzilla impostor emerges from Mount Fuji, leaving behind a trail of destruction. Godzilla’s ally, Anguirus, confronts the impostor but is wounded and forced to retreat. Keisuke arrives to check on Masahiko and Miyajima, and the rampage intensifies as the real Godzilla clashes with the impostor. The intruder is revealed to be Mechagodzilla, a towering mecha forged from space titanium and armed with advanced weaponry. The two behemoths hurt each other and retreat to regroup, each testing the other’s limits in a battle that could tilt the fate of the planet.
Keisuke and Saeko hurry the statue back to the temple, but the thief returns for another attempt. The stranger’s face suddenly melts away, exposing a Simeon—an ape-like alien from the Third Planet of the Black Hole—who proceeds to attack the two, only to be cut down by an unseen gunman. Nanbara, an Interpol agent, had been tracking the Simeons and briefly glimpses the confrontation before the chaos explodes into a new chapter of danger. Godzilla returns to Monster Island during a raging thunderstorm, and a bolt of lightning somehow restores the giant’s vigor.
Miyajima, Masahiko, and his daughter Ikuko explore the caves where the space titanium was found, only to be captured by Simeons who intend to use Mechagodzilla to conquer Earth. Their commander, Kuronuma [Gorō Mutsumi], forces Miyajima to repair the war machine. Saeko checks into a hotel and guards the sacred statue, while Keisuke leaves to reunite with Masahiko at the caves, where he encounters Nanbara—an Interpol agent who has been tracking the Simeons. After Nanbara and Keisuke infiltrate the Simeon base and free the prisoners, Keisuke and Ikuko depart to retrieve Saeko and the statue. Miyajima, Nanbara, and Masao Kunito [Masao Imafuku] are captured once more, deepening the peril.
The next morning brings a lunar eclipse: a red moon and a mirage of the sun rising in the west, signaling that Caesar must be awakened. They reunite with the Azumi priestess and her grandfather, placing the statue in the temple to reveal Caesar’s resting place. Kuronuma dispatches Mechagodzilla as the moon climbs higher, and Godzilla finally arrives to lend aid. The two monsters join forces to oppose the metal behemoth. When Mechagodzilla attempts to flee, Godzilla generates an electromagnetic field to lure it in, and then tears off Mechagodzilla’s head, destroying the threat. While the Simeons are momentarily distracted, Nanbara and the others free themselves, kill their captors, and sabotage the base, fleeing as it explodes in a fiery finale. With the enemy defeated, Godzilla returns to the ocean and Caesar resumes his eternal rest, bringing relief and celebration to the people.
