It introduces two giant, hairy humanoids called Gargantuas, which spawned from the discarded cells of Frankenstein's monster from the previous film and are described as brothers. The Green Gargantua is violent and savage, preying upon human beings; as he lives in sea water, he is given the name Gaira (ガイラ?, from kai, "sea"). The Brown Gargantua had been raised in captivity, and is docile and gentle; because he resides in the Japan Alps, he is called Sanda (サンダ?, from san, "mountain"). The film follows the investigation and military engagements of these creatures until their climatic confrontation in Tokyo.
Several ambiguous references are made to Frankenstein vs. Baragon, but the only direct link between the films is the term "Frankenstein", which appears in the title and is used to refer to the Gargantuas ("Frankensteins") in the original Japanese dialogue. Like the previous film, which starred Nick Adams, War of the Gargantuas features a Hollywood actor (Russ Tamblyn) in the lead as a scientist, Kumi Mizuno as his colleague, and another Japanese scientist (previously Tadao Takashima, here Kenji Sahara). The similar casting has led to speculation that the film was intended to feature recurring characters. Eiji Tsuburaya helmed the special effects crew with monster suit actor Haruo Nakajima portraying the antagonistic Gaira.
The film itself is rather vague as to where (if at all) War of the Gargantuas falls in regard to the continuity of Toho's other kaiju films, or even if it should be considered a canonical part of the Godzilla series. In 2002's Godzilla Against Mechagodzilla, however, specific reference to the Gargantuas is made, indicating that (in this variation on Godzilla continuity, in any case) War of the Gargantuas is considered by Toho to be a legitimate part of the Godzilla universe.
From within the pitch-blackness of the ocean depths, a powerful green giant lurked. Armed with unquestionable strength and a bloodlust matched by no other, Gaira showed itself to be a murderous creature. Acting like the right hand of the devil, the ape like creature not only defeated the Giant Octopus, but attacked the very ship that the sea beast was just latched upon. Shoving the vessel beneath the waves, Gaira devoured most of the crew members that managed to flee the boat. Like rats from a sinking ship, they paddled their way through the waves. Unfortunately their efforts would be in vain, as the beast swam after them hungrily. But the monster's appetite called for more than just a few humans on the sunken ship, and Gaira made his way to land.
First arriving at an airport, the sea beast made its presence known to the mainland. After eating a helpless woman he snatched up, Gaira finally retreated as the sun pulled itself from the darkened clouds. Mulling over the destruction, the Japanese military went into action shortly after and set a trap for the beast. Waiting for the right time, the JSDF soldiers lured the creature deeper and deeper into the mainland. Electrified with over a million volts of power, the monster seemed to be dying, that is until his brother Sanda intervened.
Apparently the second offspring of the giant Frankenstein monster, Sanda was discovered as a child living in the forest by Doctor Paul Stewart. The creature was taken to a research laboratory where he quickly befriend one of the doctor's aids: Akemi. Though Sanda was affectionate toward his captors, he eventually escaped back to his mountain home, where he grew to enormous proportions. He would not reappear until a while later, when he rushes to the aide of his brother, Gaira who is under attack by the JSDF.
However, Sanda later discovers his brother does not share his compassion toward humanity, after catching him in the act of devouring some people. The brown giant attacks his brother in anger, driving him out of the forest. Gaira retreats straight to Tokyo, traveling through the dark waters of its harbor. Sanda follows his brother, and attempts to convince him to end his bloodthirsty ways. Gaira pays no heed and the two engage in battle destroying and leveling much of the city before dying in the volcanic aftermath.
This is the original Japanese version of this trailer
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WAR OF THE GARGANTUAS (TRAILER)
ISHIRO HONDA - TOHO - (1966)