Godzilla: Tokyo S.O.S. is the sequel to Godzilla Against MechaGodzilla and is the only one in the millenium series.
The giant snapping turtle Kamoebas is a direct reference to another film made by TOHO called Space Amoeba. Originally, a Liopleurodon was considered to be used, but was switched out for one of TOHO's more popular monsters, Anguirus. The idea was scrapped because it wouldn't sit well with the fans that a popular monster would be turned into a corpse so a less popular monster was used instead.
At the end credits bonus scene, it shows a canister that holds the DNA of the 1954 Godzilla and possibly other monsters surrounding it. This was hinted of a third movie that was scrapped. It would have been a stand alone film only about MechaGodzilla and it did not get as far as a draft of a script that was submitted to TOHO.
It has been a year since Godzilla's defeat and MechaGodzilla is under repair. However, Mothra warns of an all out war against humanity if Godzilla's bones are not returned.
Continuing where Godzilla Against MechaGodzilla left off, Godzilla: Tokyo S.O.S. is another one of the better Godzilla films of the millenium series. Just like the previous movie, it is more story focused and the story is probably as best as it could get in this series, which doesn't say much. It goes straight to the point and it's easier to follow. Also, since this is in the same time line as the movie, Mothra, it gives you a quick explanation on what happened in that movie. This was so you don't have to go and watch that movie before watching this one. Despite being easier to follow, the characters are sort of boring which makes it hard to like this movie.
There is not much to say about the fight scenes. It is a little better than the previous movie that it gives you what it you were looking for from Godzilla Against MechaGodzilla with Mothra tossed into the mix. You know, with Mothra threatening to wage war on mankind I would have liked to see Mothra actually pose a threat. It would make Mothra terrifying for once and it would give her some struggles about whether her decision is really the right thing or not. Instead, you get your usual Mothra trying to save the world cliche.
Godzilla: Tokyo S.O.S. made only minor improvements over Godzilla Against MechaGodzilla, but that doesn't say much. Even though it's more watchable than the previous movie it still falls short in the end.