My word, this may be the goofiest Godzilla movie since Big G faced off with Hedorah in 1971. And that's saying something, since in the intervening decades he's faced a black hole white hole space crystal version of himself, and a giant mutant rose.
In a break with the tradition of the previous few films, this film does not reset the timeline. Instead, it is a sequel to the previous year's Godzilla vs Mechagodzilla, though it switches the focus to a new group of characters working within the "Mecha G" program. Well, mostly to a new character: one of the mechanics who services the giant robot.
This mechanic's uncle encountered Mothra forty years earlier, and Mothra's two diminutive fairy spokespeople suddenly contact the uncle again while the mechanic is visiting. They bring a warning: in using the remains of the original Godzilla as the skeleton of Mechagodzilla, humanity has transgressed against the natural order. The bones must be returned to the ocean.
Naturally, the suggestion that they scrap the Mechagodzilla project doesn't go over well with the authorities, when they hear it. Nor is our protagonist a fan of the idea. Not even the promise that Mothra will defend Japan if they do is enough to change their minds (and to be honest, I doubt it would mine, either: what's a giant moth going to do against Godzilla, eat his clothes?).
Of course, Big G himself soon turns up for his regularly scheduled rampage. The army is mobilised to defend Japan but Mechagodzilla is not fully repaired yet, so they hold it in reserve. Godzilla seems to be able to sense the robot's presence though, as he makes a beeline toward it, easily brushing the armed forces aside.
Fortunately, the mechanic's nephew and uncle manage to summon Mothra to the battle. This slows Godzilla's advance, but it soon becomes apparent that Mothra is outmatched. Now, she's quite the planner and has an egg in reserve, but that's not going to be much help right this second, so the Japanese Prime Minister orders Mechagodzilla to go to her aid.
All of which sounds pretty straightforward, and it is. It even remains so when Mothra's egg hatches and two silk worms emerge (just like in the original Mothra vs Godzilla) and cocoon Big G (also just like in the original Mothra vs Godzilla). It's only after all that happens that the film gets on the goofy-train. I won't spoil the details: they're probably better experienced than described.
Overall, this is one for kaiju fans only.