The following year, Toho/UPA produced a sequel to the film titled The War of the Gargantuas. The film was released theatrically in the United States in the summer of 1966 by American International Pictures.
The prologue is set in Nazi Germany during the final days of World War II. A Kriegsmarine Officer, flanked by three Commandos, barges into the laboratory of a Dr. Riesendorf with orders to seize the immortal heart of the Frankenstein Monster, on which Riesendorf is busy experimenting. The heart is summarily transported by U-Boat to be passed off to their Japanese allies via the Atlantic. In the Indian Ocean, off the Maldives, the U-Boat meets up with a Japanese Imperial Navy submarine to make the exchange. They are sighted by an Allied Forces scout plane and bombed, but not before the Kriegsmarine pass the heart (contained in a locked chest) to the Japanese, who take it back to Hiroshima for further experimentation. But just as the experiments are about to begin, Hiroshima is bombed with a nuclear weapon by the Allied Forces, and the heart and the experiments vanish in the atomic fireball.
Fifteen years later, in 1960, a feral boy runs rampant in the streets of Hiroshima, catching and devouring small animals such as dogs and rabbits. This comes to the attention of American scientist Dr. James Bowen and his assistants Sueko Togami and Ken'ichiro Kawaji.
The Former Imperial Navy Officer Kawai, who brought the heart of Frankenstein's Monster to Japan in WWII, is now working in an oil factory in Akita Prefecture, when a sudden earthquake shakes the very foundations of the refinery and an offshore drilling tower collapses. As the ground splits open, Kawai, for a moment, glimpses a monstrous, inhuman visage peering through the fissure, and an unearthly glow, before it is obscured by collapsing wreckage.
While Frankenstein is on the run, he travels to many places, from Okayama (where he eats more animals) to Mount Ibuki, where his primitive childlike activities (throwing trees at birds and trying to trap a wild boar) end in disaster.
Bowen, Sueko, and Kawaji then form a search party and venture into the forest in which they believe Frankenstein is hiding. But Kawaji, to the shock of Bowen and Sueko, then proceeds to attempt to kill him, believing that Frankenstein could be dangerous by his very nature, and not even Sueko could possibly tame him. He intends to blind him with chemical grenades and capture him to recover his heart and brain. Kawaji presses on to find Frankenstein, and instead finds Baragon. Kawaji and Bowen try in vain to stop the monster with the grenades, and when it is about to eat Sueko, Frankenstein comes to the rescue. The cataclysmic battle between the two giant monsters then begins. After the fight, the area where the fight took place starts to tremble, and then both monsters are sucked into the earth.
- Nick Adams as Dr. James Bowen (Japanese voice actor: Goro Naya)
- Tadao Takashima as Dr. Kawaji
- Kumi Mizuno as Dr. Sueko Togami
- Yoshio Tsuchiya as Captain Kawai
- Yoshifumi Tajima as Murata
- Takashi Shimura as Hiroshima surgeon
- Susumu Fujita as Osaka Police Chief
- Peter Mann as Dr. Riesendorf (Japanese voice actor: Kazuo Kumakura)
- Keiko Sawai as Tazuko Tooi
- Koji Furuhata as Frankenstein
- Sumio Nakao as Younger Frankenstein
- Haruo Nakajima as Baragon
Toho had always been interested in the Frankenstein character as, in 1961, producer Tomoyuki Tanaka commissioned a film project called Frankenstein vs. the Human Vapor (フランケンシュタイン対ガス人間 - Furankenshutain tai Gasu Ningen). Acting as a sequel to the 1960 film The Human Vapor, the Mizuno character from that film finds the Frankenstein Monster's body, and revives him, so that he can help him use the Frankenstein formula to revive his beloved girlfriend Fujichiyo (who had died at the end of said film). As a rough draft of the story was being written by Kaoru Mabuchi (a.k.a. Takeshi Kimura), it was ultimately cancelled before the draft was finished.
In 1962, Toho purchased a script from an independent producer from America named John Beck called King Kong vs Prometheus. Beck had stolen the story treatment (which was originally called King Kong meets Frankenstein) from Willis O'Brien and had George Worthing Yates flesh it out into a screenplay. Toho wanted to have King Kong fight their own monster Godzilla instead of the Frankenstein giant in the original story and, after working out a deal with Beck as well as RKO, the copyright holder of King Kong at the time, produced King Kong vs Godzilla.
Influenced by the concept of the giant Frankenstein monster from the King Kong meets Frankenstein/King Kong vs. Prometheus story, Toho planned on making Frankenstein vs. Godzilla (フランケンシュタイン対ゴジラ - Furankenshutain tai Gojira) as a follow up to King Kong vs. Godzilla. Written in 1963 by Takeshi Kimura (using the pen name Kaoru Mabuchi), the story dealt with the heart of the original Frankenstein monster becoming irradiated and growing into a Frankenstein-monster giant. Afraid the giant would start eating people, Godzilla would be freed from an icy prison by the JSDF and goaded into a fight with the monster in hopes of killing him. Even though King Kong vs. Godzilla had already been made with Godzilla escaping from an iceberg that he was trapped in at the end of Godzilla Raids Again, script writer Mabuchi thought with Godzilla disappearing into the ocean at the end of that film, that the idea of Godzilla becoming frozen in the North Sea into another icy prison could still work. The story would end with natural disasters defeating the monsters as Godzilla disappears into a raging river flow, and the Frankenstein giant disappears into magma caused by an erupting volcano
In 1965, they would finally co-produce the story with financial backing from Henry G. Saperstein's film company Henry G. Saperstein Enterprises into this film. A new dinosaurian opponent named Baragon was created to replace Godzilla as Frankenstein's opponent, and the script was slightly altered. Most of the concepts from the original story treatment were retained in this version such as the irradiated heart of the monster, the monster's relentless pursuit of food, and a natural disaster defeating the monster during the climax. In addition, most of the characters from the original story such as research scientist Dr. Bowen (played by Nick Adams), would be retained. In the US version, Jerry Sohl would get credit for a synopsis and executive producer Reuben Bercovitch would get credit for the story.
Koji Furuhata earned the role of Frankenstein's monster through an open audition. Furuhata wore green contact lenses to emulate a Caucasian look, a flat-head prosthetic and brow resembling Jack Pierce's Frankenstein design, and large shirts and loincloths. Honda had originally wanted to explore more of the science-gone-wrong theme but was forced to change the story in the middle to reach climactic monster battle.
Henry G. Saperstein had requested an alternative ending for the international release in which Frankenstein battled a giant octopus. This resulted in the cast and crew being reassembled after principal photography and post-production had wrapped, as well as building a new set and creating the octopus. Despite filming the new ending, Saperstein ended up cutting it regardless because he believed the octopus "wasn't that good".
When the film was in production, trade magazines listed this film's title as Frankenstein vs. The Giant Devilfish. Honda had stated the reason why the octopus ending was initially requested was because the American co-producers were "astonished" by the octopus scene in King Kong vs. Godzilla and wanted a similar scene. Honda also confirmed that various endings were shot, stating: "In fact Mr.Tsuburaya had shot five or six final scenes for this film. The infamous giant octopus is only one of these endings." Honda also expressed that the alternate ending was never intended to be released on the Japanese version, sating: "there was never any official plan to utilize the sequence; but an alternative print with that ending was accidentally aired on television surprising many Japanese fans because it was not the ending they had remembered from the original theatrical release."
When Benedict Pictures would co-produce the film's sequel The War of the Gargantuas with Toho the following year, an octopus sequence would be shot again (a marine-based battle between the Oodako and Gaira) that would remain intact in both versions of the film.
The following year, Toho released a sequel titled The War of the Gargantuas, also co-produced with UPA. In the film, pieces of Frankenstein's cells mutate into two giant humanoid monsters: Sanda (the Brown Gargantua) and Gaira (the Green Gargantua). The former is a benevolent and peace-loving creature, the latter is murderous and savage. Apart from a reference to a severed hand, UPA obscured all references to Frankenstein in the American version and the names of the monsters were changed to Brown Gargantua and Green Gargantua. Gaira and Sanda would later appear in two of Toho's Tokusatsu series, Ike! Godman and Ike! Greenman, before remaining absent for over 40 years, with Gaira making his latest appearance in a 2008 Ike! Greenman special.
The film was released in Japan on August 8, 1965, 2 days after the 20th anniversary of the Hiroshima bombing. The film grossed ¥93 million during its Japanese theatrical run. The film was released a year later in the United States as Frankenstein Conquers the World via American International Pictures, with Adams' original English dialogue restored. This version was dubbed in English by Titra Studios.
In June 2007, Media Blasters released Frankenstein Conquers the World on a 2-disc DVD, the first time the film was released on DVD in North America. This version includes the original Japanese theatrical version, the original US English version from American International Pictures (running at 84 minutes), and the International version with the alternate octopus ending (running at 93 minutes). All three versions in widescreen. The international version (titled Frankenstein vs. The Giant Devilfish) features an audio commentary by Sadamasa Arikawa, the film special effects photographer. Disc two features two Japanese trailers, deleted scenes, and a photo gallery, which was provided by Ed Godziszewski (editor of Japanese Giants and author of The Illustrated Encyclopedia of Godzilla.)
In November 2017, Toho released the film on Blu-ray in Japan. This release also includes an HD remaster of the International version, Frankenstein vs. The Giant Devilfish.