An underground nuclear test in the Aleutian Islands has widespread environmental effects, even as far away as Monster Island in the South Pacific. Godzilla and the other monsters are in distress and attempt to escape the earthquakes, volcanic eruptions, and storms.
The effects are also felt in Japan, where inventor Goro Ibuki, his son Rokuro, and Goro’s friend Hiroshi Jinkawa are enjoy a day at the lake. An earthquake creates a crack in the lake bed, draining the water underground. The day ruined, they return to Ibuki’s home, where the door is unlocked and open. They are attacked by several mysterious men, who flee. Hiroshi gives chase, but they manage to escape. The attackers were apparently trying to find information about the robot Ibuki is building. The leave behind an unusual red sand, which is revealed to have come from deep undersea.
Now completed, the robot is activated and named “Jet Jaguar.” The mysterious agents snatch Rokuro and use him as ploy to get into the house, using a sleep gas to knock out the two men and the boy. Receiving orders from the Emperor of Seatopia, they program the robot to lead Megalon to “the target.”
At the undersea kingdom of Seatopia, Emperor Antonio informs his subjects they are going to war against the Earth because the nuclear testing has already destroyed a third of the country. He unleashes the giant monster Megalon. The giant winged beetle with a glowing antenna and sharp, serrated claws rises to the surface.
A Seatopian agent has tied up Ibuki and his son in the back of a truck and is heading toward the cracked lake, with plans to take them to the undersea kingdom. Back at the house, Hiroshi breaks free from his bonds, knocks out a Seatopian agent, and escapes to rescue the others.
Megalon arrives at the lake, meeting Jet Jaguar who leads him to Tokyo. The city is being evacuated, and army units are being sent in. The truck drivers throw the Seatopian out of the cab, and decide to dump the container with Ibuki and his son into the dam instead of going to the lake. Megalon interrupts their task and they run off. The monster smashes the dam, releasing the water into the valley below. The container slips off the truck, but Megalon bats it over the mountain. The doors crack open spilling out the pair.
The army is making a valiant attempt to stop Megalon before he gets to Tokyo, but the beast breaks through the line and uses a laser emitting from his antenna to destroy the defenders. In an army helicopter, Ibuki uses the transmitter to override the Seatopian programming and sends Jet Jaguar to get Godzilla. Realizing the King of the Monsters will arrive soon, the Seatopians request assistance from Space Hunter Nebula M, who send Gigan.
Megalon arrives in Tokyo and uses his antenna laser to destroy huge sections of the city. Jet Jaguar meets with Ibuki and reports that Godzilla is on the way. However, Ibuki no longer controls the robot – it has reprogrammed itself “for survival.” It flies to meet Megalon, and grows to match the monster’s size in order to do battle.
Jet Jaguar is barely able to hold his own against Megalon, when Gigan appears. Caught between the two monsters, the robot tries to fly off. But Megalon knocks him out of the sky with his antenna laser. On the ground, the beasts knock Jet Jaguar around. Just as it appears hopeless, Godzilla arrives and throws some kaiju karate moves on Megalon and Gigan while Jet Jaguar licks his wounds.
Both of the monsters are seriously injured and lying on the ground. Gigan manages to get back up, flies toward Godzilla, and slashes Godzilla in the shoulder with the buzz-saw blade in his abdomen, This enrages Godzilla, who blasts the space monster with his nuclear breath, knocking it out of the sky. Gigan threatens to decapitate Jet Jaguar, but Godilla blasts Gigan again, who retreats to Megalon. The creatures surround Godzilla and Jet Jaguar with a ring of fire, but the robot lifts Godzilla out of the inferno. The allies thrash Megalon and Gigan, causing Gigan to flee like a wounded bully back to Space Hunter Nebula M. Godzilla and Jet Jaguar continue to beat on Megalon, who falls into a crevice that leads back to Seatopia.
Defeated, the Emperor orders all exits to the surface closed. Godzilla returns to Monster Island. Jet Jaguar shrinks to human size and returns to his creator.
Human Cast: Katsuhiko Sasaki (Goro Ibuki), Hiroyuki Kawase (Rokuro Ibuki), Yutaka Hayashi (Hiroshi Jinkawa), Robert Dunham (Seatopia Emperor Antonio)
Monster Cast: Godzilla, Megalon, Jet Jaguar, Rodan, Angirus
Notes: Jet Jaguar was based on a design created by a school child as part of a Toho-sponsored contest. He was originally intended to be in his own movie, but studio execs didn’t think he could stand alone. The English language translation pronounces the robot’s name as “Jet Jag-you-are.” While there is no “official” DVD release of this movie in North America, it does show up on TV from time to time, and there is a Mystery Science Theater 3000 version on DVD.
LogBook entry by Robert Parson