During a ceremonial dance on Okinawa, a woman faints after experiencing visions of monsters destroying a city. Masahiko, a reporter sent to cover the ceremony, later visits a cave and discovers a radioactive metal, which later analysis reveals is Space Titanium. Meanwhile, his brother, who is developing a marina, finds another cave containing ancient artifacts, apparently honoring an ancient god, King Caesar. Paintings on the wall predict an attack by a monster that will destroy the world, but two other monsters will save the world.
Mainland Japan is being rocked by a series of earthquakes, with a “moving epicenter,” and Godzilla emerges from a volcanic eruption at Mount Fuji. His roar, however has a strange metallic sound. As he begins his rampage, Anguirus attacks. The two battle, but Godzilla seriously wounds the spiked creature, who retreats to fight another day. Godzilla marches off. At the scene of the battle more of the Space Titanium is found.
Godzilla is crashing through an industrial sector in Tokyo, destroying oil storage tanks and pipelines. Suddenly, a second Godzilla appears. The two thrash around, destroying more of the industrial sector, when the skin is ripped from one of the Godzillas. It is a cyborg made of Space Titanium, and is controlled by aliens. MechaGodzilla launches missiles from its fingertips and lasers from its eyes. Godzilla uses his nuclear breath. Both are injured. Godzilla retreats to the ocean and MechaGodzilla is recalled by his controllers.
While investigating the cave on Okinawa, Professor Miyajima, his daughter, Eiko, and Masahiko are grabbed by the ape-like aliens from the “Third Planet of the Black Hole,” who have taken human form. They demand that Miyajima help repair MechaGodzilla, and threatens to kill the others unless he complies. Meanwhile, on a remote island Godzilla is recovering from his wounds.
Miyajima has finished his repairs to MechaGodzilla. The alien controller throws him into the execution room with Eiko and Masahiko. The room is filled with hot, pressurized steam. Keisuke and an Interpol agent, who has been investigating the aliens, break into the stronghold and rescue the others from the deadly steam.
King Caesar awakens from his ages-long sleep. The aliens send MechaGodzilla after the giant creature, which looks like a giant floppy-eared lion-dog cross with a menacing toothy grin in a stone face.
MechaGodzilla unleashes a laser blast at King Caesar, but the ancient god reflects it back. Caesar hides behind a giant rock when the metal monster fires his finger missiles. The two continue to grapple and Caesar is tossed aside like rag doll, when Godzilla arrives. The King of the Monsters and MechaGodzilla battle while King Caesar recovers. Caesar pulls himself up and MechaGodzila launches a full attack with all of its ordnance. Godzilla is seriously wounded, but manages to shake it off. With new magnetic abilities, Godzilla pulls the metal monster toward him. Godzilla grabs it from behind and Caesar begins smashing at it from the front. Godzilla rips the head off the cyborg, defeating it.
As the alien base explodes, Godzilla returns to the sea and Caesar returns to his slumber.
Human Cast: Masaaki Daimon (Keisuke Shimizu), Kazuya Aoyama (Masahiko Shimizu), Akihiko Hirata (Professor Hideto Miyajima), Hiromi Matushita (Eiko Miyajima)
Monster Cast: Godzilla, MechaGodzilla, King Caesar, Anguirus
Notes: The city-destroying vision the young woman has at the beginning the movie features King Ghidorah, who is otherwise not seen in the movie.
LogBook entry by Robert Parson