In the year 1973, the most recent underground nuclear test, set off near the Aleutians, sends shockwaves as far south as Monster Island, disturbing the monsters, causing Anguirus to fall/burrow into a fault opened up by the consequential earthquakes and Rodan to fly off, while Godzilla decides to stay put.
For years, Seatopia, an undersea civilization, has been heavily affected by this nuclear testing conducted by the surface nations of the world. Upset by these tests, the Seatopians plan to unleash their civilization's beetle-like god, Megalon, to destroy the surface world out of vengeance.
On the surface, an inventor named Goro Ibuki, his brother Rokuro and their friend Hiroshi Jinkawa are off on an outing near a lake when Seatopia makes itself known to the Earth by drying up the lake the trio was relaxing nearby and using it as a base of operation (unknown to our heroes.) As they return home they are ambushed by agents of Seatopia who are trying to steal Jet Jaguar, a humanoid robot under construction by the trio of inventors. However the Agents' first attempt is botched and they are forced to flee to safety.
Some time later, Jet Jaguar is completed but the trio of inventors are knocked unconscious by the returning Seatopian agents. The agents's plan is to use Jet Jaguar to guide and direct Megalon to destroy whatever city Seatopia commands. Goro and Rokuro are sent to be killed, while Hiroshi is taken hostage. Megalon is finally released to the surface while Jet Jaguar is put under the control of the Seatopians and is used to guide Megalon to attack Tokyo with the Japan Self Defense Forces failing to defeat the monster. Eventually, the trio of heroes manage to escape their situation with the Seatopians and reunite to devise a plan to send Jet Jaguar to get Godzilla's help using Jet Jaguar's secondary control system.
After uniting with Japan's Defense Force, Goro manages to regain control of Jet Jaguar and sends the robot to Monster Island to bring Godzilla to fight Megalon. Without a guide to control its actions, Megalon flails around relentlessly and aimlessly fighting with the Defense Force and destroying the outskirts of Tokyo. The Seatopians learn of Jet Jaguar's turn and thus send out a distress call to the Nebula M aliens (from the previous film) to send Gigan to assist them.
As Godzilla journeys to fight Megalon, Jet Jaguar programs into a safeguard mode and grows to gigantic proportions to face Megalon himself until Godzilla arrives. The battle is roughly at a standstill between robot and monster, until Gigan arrives and both Megalon and Gigan double team Jet Jaguar. Godzilla finally arrives to assist Jet Jaguar and the odds become evened. After a long and brutal fight, Gigan and Megalon both retreat and Godzilla and Jet Jaguar shake hands on a job well done. Godzilla returns to Monster Island, and Jet Jaguar returns to his previous, human-sized state and reunites with his inventors.
- Katsuhiko Sasaki
- Hiroyuki Kawase
- Yutaka Hayashi
- Robert Dunham
- Kotaro Tomita
- Wolf Ohtsuki
- Gentaro Nakajima
- Sakyo Mikami
- Filmed in three weeks and originally planned as "Jet Jaguar vs Megalon" before Godzilla and Gigan were added for extra star power.
- Jet Jaguar is the result of a children's monster design contest that Toho held in 1972. The winning entry used as the basis of his design was named "Red Alone" and had a much more birdlike head and a pair of wings.
- The only Godzilla film to receive a television premiere on a major U.S network, as NBC aired it on prime time television in the summer of 1977, where it was hosted by actor John Belushi dressed in a Godzilla costume.
- Many of the scenes are comprised of stock footage from other Toho films, primarily Godzilla vs. Gigan which was released a year earlier. The mobilization sequence is comprised of footage from Invasion of Astro-Monster, The Mysterians, War of the Gargantuas, and Destroy All Monsters. The shot of Godzilla's shoulder being sawed open was also from Godzilla vs. Gigan, and you can clearly see that Godzilla in that scene looks different from Godzilla in the rest of the movie.
- The scene that shows Megalon arm swatting planes out of the sky was actually stock footage of Gigan's hooked claw destroying planes from Godzilla vs Gigan.
- Megalon's ray beam is animated the same way as King Ghidorah's gravity beams. This was made to ensure that stock footage of the destruction of Tokyo from Ghidorah: The Three Headed Monster and Monster Zero films could easily be interspliced between shots of Megalon firing his beam.
- The destruction scenes also include footage from Godzilla vs. Hedorah, with Megalon's ray beam added, and completely original destruction footage filmed for Godzilla vs Gigan. Some of the scenes with what is supposed to be Megalon are actually Gigan. And Megalon's ray is actually King Ghidorah's Gravity beams destroying buildings.
- This film was originally produced in 1973 but was not released in the United States until 1976. A small US distributor, Cinema Shares, hoped to ride the promotional wave of Dino De Laurentiis's King Kong. This became especially apparent in the US poster art, which depicted Godzilla and Megalon battling each other on top of the World Trade Center towers.
- Godzilla vs Megalon was originally released briefly by Cinema Shares almost uncut. Eventually, more cuts were made, to keep a "G"-rating from the MPAA. Edits include
- The removal opening credits, except for the title and a "Produced by Toho-Eizo Co., Ltd." credit.
- Rokuro being abducted by Seatopian agents, who pull him into their car.
- The Seatopian agents knocking out Rokuro and barging inside.
- Scenes in the container truck that showed pin-ups on the back wall. They are Playboy centerfolds of Deanna Baker and Gloria Root.
- A scene of the bearded Caucasian Seatopian agent being thrown down a cliff by the truck drivers.
- The lead Seatopian is hit in the face with the model jet, causing him to bleed.
- The same Seatopian agent is crushed by a boulder hurled by Megalon.
- In the German version of the movie, the robot Jet Jaguar is called King Kong. According to rumors, they even claim that he is supposed to be the giant ape wearing a robot suit, but that is false. The dub merely calls him King Kong, but no reference is made to him being anything other than a human-built robot. The reason for this is that the distributors probably wanted to ride the popularity of the King Kong brand. Curiously, the evil robot Mechagodzilla from the following movie, who has no connection to the benevolent Jet Jaguar, is also called King Kong in Germany.