Godzilla: Tokyo S.O.S. is the first, and only, Godzilla film in the Millenium series to be a sequel to another film the Millenium series, namely, the previous film, Godzilla Against Mechagodzilla, with the two films being known as the Kiryu Saga by fans. Known in Japan as Gojira tai Mosura tai Mekagojira Tōkyō Esu Ō Esu.
After Godzilla's defeat in the previous film, Japan is warned by Mothra's twin fairies, the Shobijin, to put Kiryu to rest in the sea, as the bones of the dead should not be disturbed. The JSDF is reluctant to give up the only effective defense that they have against Godzilla, even after the Shobijin promise to have Mothra defend against Godzilla in Kiryu's stead. Not too keen on having another giant monster around, even if it's trying to help them, the JSDF refuse to retire Kiryu.
Unfortunately, Godzilla has recovered from his last battle and has claimed more victims, including a giant Kamoebas and an American submarine, and he is now heading straight for Tokyo again. When the JSDF is once again unable to stop the monster, Mothra is summoned to fight Godzilla by the grandson of an aged Professor Chujo, who's nephew works as one of Kiryu's mechanics.
When it becomes apparent that Mothra cannot hold her own against Godzilla, the JSDF launch Kiryu to support her. Back on Infant Island, Mothra's egg hatches, revealing twin larva who make their way to Japan to help their mother. Will the combined might of all four monsters be enough to stop Godzilla?
The answer is... maybe.
This film contains examples of the following:
- A.I. Is a Crapshoot: Once again, Kiryu goes rogue but only at the end of the movie and he is far more benevolent when he does so this time.
- Art Shift: Compared to her more wasp-like appearance in Godzilla, Mothra, King Ghidorah: Giant Monsters All-Out Attack!, Mothra's appearance in this film hearkens to the "fluffy butterfly" look of the Show and Heisei incarnations.
- Bare Your Midriff: The Shobijin.
- Big Damn Heroes: First by Mothra, and when Mothra's in trouble, it's Kiryu's turn again.
- Fanservice: This film's incarnation of the Shobijin sport the most revealing attire of any Shobijin.
- Haunted Technology: Again, Kiryu. This may also tie into the Shobijin's reason for not disturbing the bones of the dead, although this point is never explicitly stated.
- Heroic Sacrifice: Mothra puts herself directly in the path of Godzilla's atomic ray to protect her larvae.
- Inter Continuity Crossover: Let's see... Kamoebas from Space Amoeba? Check. Professor Chujo (played by his original actor, Hiroshi Koizumi) and Mothra herself from Mothra (1961)? Check.
- Mythology Gag: Both of Godzilla's battles with Mothra and her larva are shout outs to Mothra vs. Godzilla.
- Chujo's introduction has his face covered, much like how he was introduced in Mothra's film.
- No Plans, No Prototype, No Backup: Averted. It's not that the Mechagodzilla team aren't capable of replacing the Absolute Zero cannon, it's that they don't have the budget to commission the necessary parts.
- Put on a Bus: Akane, the pilot from the previous film, is sent overseas for further combat training around the start of the film.
- Red Eyes, Take Warning: After seeing Godzilla kill their mother, the Mothra larvae's eyes change from blue to purple, and finally to red.
- As in the previous film, Kiryu's rampant state is visibly cued by what look like glowing red tearstreaks down the sides of its head.
- Sacrificial Lamb: Kamoebas. He only shows up to prove Godzilla can kill something stronger/bigger than just humans.
- Slashed Throat: How Godzilla offed Kamoebas.
- Take That!: Godzilla killing off poor Kameobas serves as "revenge" on Gamera, who was solely made by a studio rivaling Toho to compete with Godzilla at the box office.
- The Stinger: The reveal of the 1954 Godzilla's DNA in a biotechnology research freezer vault alongside DNA samples from other kaiju.
- The Tokyo Fireball: Godzilla and Mothra both end up creating one when Godzilla uses his atomic ray while surrounded by Mothra's scales.
- This Is a Drill: Kiryu's right hand has been upgraded during repairs to be able to turn into a drill.
- Worf Had the Flu: Not only has Kiryu's Absolute Zero cannon been replaced with a tri-maser, much less effective, but it is worse at melee due to slower response time since repairs were not completed. Almost seems like a nod to Showa Mechagodzilla's weakening between films.