Megalon was a big, four-legged insect that was the god of the advanced (of course), undersea Kingdom of Seatopia. The Japanese did several underwater nuclear tests that peeved off the Seatopians. The Seatopians awoke and sent Megalon to the surface to punnish the Earthlings. The Seatopians stole the robot, Jet-Jaguar, to guide Megalon. Jet-Jaguar later grew in size and fought with Megalon. Gigan then joined in the fight. Godzilla then arrived and the four monsters duked it out. Gigan retreated back into space. Godzilla then beat-up Megalon with an infamous flying kick. Megalon retreated back into Seatopia and the Earth had been saved.

Megalon was the star of what is considered the third-worst Godzilla movie ever made. (Godzilla's Revenge is considered the second-worst, and Godzilla 1998 is considered the worst). The Japanese economy, at the time the movie was made, was very poor, and Toho was unable to give a lot of money to director Jun Fukuda. The budget for Godzilla vs. Megalon was only half of the budget of the Godzilla movies from the 1950s and 1960s. This, obviously, resulted in the movie having poor special effects. The movie was rushed into production and was shot in less than three weeks. Nevertheless, there were several memorable moments in the movie, most notabaly, Godzilla's flying kick. Godzilla's popularity decreased in the 1970s and Toho decided to star him on the T.V. show "Zonefighter," instead of making Godzilla films.

Some of the info above came from Megalon's Domain.