The film stars Katsuhiko Sasaki, Hiroyuki Kawase, Yutaka Hayashi, and Robert Dunham, alongside Shinji Takagi as Godzilla, Hideto Date as Megalon, Kenpachiro Satsuma as Gigan, and Tsugutoshi Komada as Jet Jaguar.

In the first part of 1971 (197X in the Japanese version), the second of a series of underground nuclear tests is conducted, near the Aleutians, sending shockwaves as far as Monster Island in the South Pacific, severely damaging the island paradise and sending Anguirus plummeting into the depths of the Earth, with Godzilla narrowly escaping the fissure into which its friends tumble.

For millions of years, Seatopia, an opulent undersea civilization that resides in vast cities reminiscent of those of Ancient Greece and Rome, has existed in relative peace, ruled by Emperor Antonio, but nuclear tests in recent years have severely affected the cities via the earthquakes the tests produced.

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.

Some time later, Jet Jaguar is completed but the trio of inventors are knocked unconscious by the returning Seatopian agents.

The agents' plan is to use Jet Jaguar to guide and direct Megalon to destroy whatever city Seatopia commands him to do.

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.

After a long and brutal fight, Gigan and Megalon both retreat and Godzilla and Jet Jaguar shake hands on a job well done.

[5] Contrary to popular belief, there is no evidence Godzilla vs. Megalon was originally planned as a Jet Jaguar solo film, and no Japanese sources have surfaced which claim otherwise.

The winner of the contest was an elementary school student, who submitted the drawing of a robot called Red Arone.

This suit also has different-sized back fins, a more circular visor, scales running up the back/sides of the neck and longer legs compared to the original version.

Given away at theatrical showings was a comic that told a simplified version of the film, which incorrectly named Jet Jaguar as "Robotman" and Gigan as "Borodan".

These cuts included a majority of the opening titles, several mild obscenities and some shots of barely-visible pornographic magazine clips when the main characters are kidnapped in a truck.

[8] The U.S. rights for the film eventually fell into the public domain in the late 80s, which resulted in companies releasing poorly-cropped, fullscreen VHS tapes mastered from pan and scan sources.

[23] Author Stephen Mark Rainey's critique of the film was strongly negative, published in Japanese Giants, issue four.

However, most of the criticism is of the lack of actual special effects work, as most of it consists of stock footage from previous films, including Godzilla vs. Gigan and Ghidorah, the Three-Headed Monster, but a few pieces of effects work have garnered praise, specifically a scene where Megalon breaks through a dam and the draining of the lake.

[22] One part of the film, on the other hand, has garnered almost universal praise: Godzilla's final attack on Megalon, a flying kick.

It has been called the saving grace of the film,[24] and was made famous by the mock exclamations of shock and awe displayed on Godzilla vs. Megalon's appearance on Mystery Science Theater 3000.

Despite all this, the film is also one of the most widely seen Godzilla films in the United States — it was popular in its initial theatrical release, largely because of an aggressive marketing campaign, including elaborate posters of the two title monsters battling atop New York City's World Trade Center towers, presumably to capitalize on the hype surrounding the Dino De Laurentiis remake of King Kong, which used a similar image for its own poster.

[25] The film was released numerous times in the VHS format, mostly as videos from bargain basement studios that featured the edited TV version (which was wrongly assumed to be in the public domain for many years), while PolyGram and 4 Front released the unedited version of the film in 1992 and 1998, respectively.

Some rumors have circulated that the film's original VHS releases in the States were uncut, but there is no evidence confirming or denying this.

Despite this, a manufacturing error led to several copies of the originally planned version featuring bonus content being released by accident.