The plot concerns two giant, hairy monster brothers spawned from the scattered cells of Frankenstein. One of the monsters, Sanda, is benevolent and peaceful, while his brother, Gaira, is savage and destructive (and also has an appetite for humans). Upon discovering his brother's brutal actions, Sanda, who is fond of humans, attacks his evil brother. What follows is a series of battles that eventually culminate in a showdown in Tokyo. Their fighting somehow sparks an undersea volcanic eruption, and the two are enveloped in the volcano's wrath.
Despite once again being made as a co-production between Toho and Benedict Pictures, the film never saw stateside release until 1970, where it played on a double bill with Monster Zero, which also reached U.S. shores four years late. There also exists an alternative Hong Kong Dub that corresponds to the Japanese version.
This film contains examples of the following:
- Always a Bigger Fish: The movie begins with a Japanese fishing boat being attacked by a giant octopus, until Gaira shows up and tears it to pieces.
- Axe-Crazy: Gaira is just one big, bloodthirsty brute.
- Behemoth Battle: The brown Gargantua is a good guy who likes and protects humans. The green Gargantua is violent, savage, and eats people. They are brothers who were raised separately.
- Berserk Button: Realizing that his "brother" eats humans appears to be Sanda's berserk button.
- Bigfoot, Sasquatch and Yeti: Sanda isn't exactly one, but he resembles a kaiju-sized one, and people compare him to one several times. Gaira is somewhere between this and Fish People.
- Broad Strokes: Intended to be a sequel to Frankenstein Conquers the World, but continuity between the films is rather "fuzzy". The two protagonists are completely different characters (despite the female lead being played again by Kumi Mizuno) and flashbacks and details about the previous appearance of "Frankenstein" don't exactly match up with the previous film.
- Cain and Abel: More like Abel and... bad Abel, since Gaira is just a wild clone of Sanda. The reference is still made by the film's characters.
- Cavalry Betrayal: Gaira saves the fishing ship from the giant octopus... and then finishes destroying the ship, killing and eating nearly everyone on board.
- Daylight Horror: On a cloudy day, Gaira makes landfall, and again when he attacks hikers on a mountain when it's foggy and cloudy.
- The Determinator: Sanda is determined to stop Gaira's rampage, even while suffering from an injured leg.
- From a Single Cell: Like their predecessor, Frankenstein, both of the Gargantuas can regrow into whole beings from just a piece of flesh, as Sanda undoubtedly was grown from the hand that Frankenstein lost in the previous film.
- Gentle Giant: Sanda, much like his progenitor, Frankenstein, is generally kind and compassionate, even going so far as to try to reason with Gaira during their fight in Tokyo. Gaira, however...is a different story.
- Giant Equals Invincible: Subverted. Gaira is generally impervious to most weaponry at first, thanks to his healing factor, which he inherited from Sanda and Frankenstein, but continuous fire does appear to affect him, even if only for a short while before he gets used to it. He is, however, vulnerable to maser fire and electricity, especially when there's an electric current flowing through the stream or river that's he's standing in.
- Green and Mean: Gaira
- I'm a Humanitarian: Gaira, for some reason, decides that humans would make a better meal than the giant octopus that he just defeated. It's a bit worse when one takes into account that Gaira and Sanda are born from Frankenstein, and essentially mutated humans, making Gaira a cannibal.
- Motherly Scientist: Akemi was this to the young Sanda, like her predecessor, Dr. Sueko Togami (also played by Mizumo) was to Frankenstein.
- Nature vs. Nurture: Gaira is Nature while Sanda is Nurture.
- No Pronunciation Guide: Gaira was oddly romanized as Gaillah and pronounced as Gaiya in the export dub and all versions based on it.
- Primate vs. Reptile: Played with. Technically given their origins, Sanda and Gaira are both primates. However, Sanda is more mammalian in appearance and personality while Gaira is green, almost completely scale-covered, and behaves like a giant reptile, i.e. terrorizing and eating people. As the title implies, they fight.
- Recycled Soundtrack: While this was common for Ifukube, especially with his tight schedules, here it seems almost deliberate, as both Frankenstein and Baragon's themes are reused as a way of reinforcing the connection between the two films.
- Sibling Rivalry: Gaira and Sanda's differing views of humans is what leads to the titular war between them.
- Weakened by the Light: Gaira, but subverted in that he can still walk around when it's cloudy outside, although he prefers the dark.
- Worf Had the Flu: Interestingly, both opponents are not at full strength for the finale. Gaira has numerous maser burns while Sandra is having trouble moving from a broken leg.