Watched it.
Giant monsters start showing up from the end of the 20th century and wreck Earth, the worst of all being Godzilla. Also aliens show up and befriend mankind and help fight the monsters but by the mid 21st century 90% of the population is dead anyway and mankind has to abandon Earth.
20 years later and one of the two immigrant ships is running out of resources with no habitable planet in site and they decide to warp back to Earth, especially since protagonist Haruo has been researching Godzilla and has a plan on how to kill him (turns out that Godzilla has a shield which is what let him do shit like tank 150 nukes at the same time and Haruo has figured out how to turn it off).
They go back to Earth where 20000 years have passed and send out a team to kill Godzilla. The team gets wrecked by the smaller flying monsters and they decide to evacuate but run into Godzilla (50 m) and carry out Haruo's plan instead. It succeeds and they kill Godzilla.
The professor then tells Haruo that what they killed couldn't have been the original Godzilla because it was completely identical to their 20000 year old data. The original Godzilla (over 300 m) then pops out of a mountain, shoots down the landing ships by barking supersonic waves at them, and then wipes out everything on the ground by waving his tail at them which shoots out a beam shockwave.
Post-credits scene has Haruo wake up after having been rescued by some indigenous girl (descendant of survivors from 20000 years ago) wearing weird junk. Final shot is a still of him and other people standing in front of Mecha Godzilla.
