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 final showdown in Tokyo. Their fighting somehow sparks an underground 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:
- 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: Seeing human beings eaten alive by his "brother" appears to be Sanda's berserk button.
- Cain and Abel: More like Abel and... bad Abel, since Gaira is really just a wild clone of Sanda. The reference is still made, however, by the film's characters.
- 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...
- 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.
- I'm a Humanitarian: Gaira, for some reason, decides that humans would make a better meal than the giant octopus that he just defeated.
- Motherly Scientist: like Akemi's predecessor (also played by Mizumo), she's far motherly to the young Sanda.
- No Pronunciation Guide: Gaira was oddly romanized as Gaillah and pronounced as Gaiya in the export dub and all versions based on it.
- Recycled Soundtrack: While this was common for Ifukube, especially with his tight schedules, here it seems almost deliberate, as both Baragon and Frankenstein's themes are reused as a way of reinforcing the connection between the two films.
- Sequel Displacement: while both are fairly obscure outside of kaiju fandom, this one gets much more play on television than Frankenstein Conquers the World...which can make the backstory about Frankenstein dying at the top of Mount Fuji somewhat confusing to first time viewers.
- Weakened by the Light: Gaira, but subverted in that he can still walk around when it's cloudy outside, although he clearly 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 an ankle injury.