Godzilla vs Megalon (1973) might have been better entitled Godzilla & Jet Jaguar vs Megalon & Gaiman, Sans Referee!, or something similarly silly. The movie’s plot is awful: a nuclear test has damaged part of unknown underwater civilization Seatopia (“they’ve even built their own Sun!”). In retaliation, the Seatopians steal the just-developed flying robot Jet Jaguar and use it to release and control their monster, Megalon, who proceeds to stomp on a city or two.
The humans regain control of Jet Jaguar and send him off to Monster Island to summon Godzilla to the war, while the Seatopians summon Gaiman from another dimension. Since Godzilla has to do some traveling to reach the battleground, Jet Jaguar returns to find Megalon and inflates to the monster’s size, but when Gaiman appears and teams up with Megalon, he’s in trouble until Godzilla finally reaches them.
Then the stomping begins.
Yeah, it’s awful. I mostly watched this while making meals or trying to wake up in the mornings. It. Is. Terrible. Nothing to recommend it at all.