In an icy North American region, an unknown aircraft is shot down by an American fighter jet. The aircraft crashes and its cargo, a low-level atomic bomb, explodes. The resulting cataclysm awakens a giant, prehistoric monster called Gamera, who has the appearance of a giant turtle with teeth and large tusks. Japanese scientists on an expedition (including Dr. Hidaka, Kyoko, and Aoyagi) nearby are given a "devil stone" by an Eskimo chieftain, who explains that the creature is called Gamera.
Gamera destroys the American jet with his fire breath and escapes into the sea. The monster heads to Japan and surfaces from Sagami Bay, where Toshio, a boy releasing his own pet turtle, sees him. Gamera destroys the city of Fujisawa and a lighthouse. However, he also rescues Toshio from falling from that same lighthouse and then retreats back into the sea. Scientists and government officials hold a conference to discuss killing the monster.
Gamera destroys a research ship, kills the crew and then heads to Tokyo. He is attacked with freeze bombs and blown up, falling onto his back as a result.
Toshio and his family decide to stay with an uncle in Tokyo because they have nowhere else to go. Toshio explains to the professor that Gamera is lonely and like regular turtles, he is not evil. Dr. Hidaka, meanwhile, has observed that Gamera consumes fossil fuels, and may seek out atomic bombs for the energy they provide. He also emits radio signals. This leads the Japan Atomic Energy Commission to figure out what to do with its stockpiles. Meanwhile, disasters and accidents start to occur: Koto Ward is struck by flash floods and ships collide in Tokyo Bay. Dr. Hidaka claims that Gamera has caused these accidents because he is hiding in the bay.
An international scientific conference is called, and they decide to use "Z Plan" based at Oshima Island, involving a consortium of American, Soviet and Japanese scientists, to eradicate Gamera. Before any action can be taken, however, Gamera lands at Haneda Airport, destroying the control tower, and proceeds to wreak havoc in Tokyo (including destroying the Tokyo Tower). Toshio and his family evacuate again, but Toshio disappears.
In the meantime, Z Plan is completed and Toshio sneaks on board the ship delivering supplies to Oshima. Gamera is lured to Oshima by lighting an oil slick path from Tokyo to the island, but an oncoming typhoon blows the fire out. Aoyagi starts a bonfire and Gamera makes his way to a volcano erupting on the island. The next day, Z Plan is put into action: Gamera is lured to a rocket and blasted off to Mars. The worldwide announcement of success extols the triumph of science over ideology, Aoyagi and Kyoko go off together and Toshio decides he is not sad, because he is going to be a scientist so he can go visit Gamera one day.
Director Noriaki Yuasa initially began production on a film titled Dai gunju Nezura (lit. The Great Rat Swarm) which would involve real rats crawling over miniatures of cities. The rats received for the film had fleas which halted production on Dai gunju Nezura. The success of Toho's Godzilla films led to the rival company Daiei to create a monster series of its own. As the miniatures for the film were already built, Masaichi Nagata had the idea to develop a giant monster to attack the city and had the idea for a giant flying turtle. Yuasa and screenwriter Nisan Takahashi developed the idea into the 1965 film Gamera the Giant Monster. Special effects technician Ryosaku Takayama designed the turtle suit used in the film.
The films created by Daiei were aimed at younger children than the Godzilla films of the period and had lower budgets. Gamera the Giant Monster was the only film in the series shot in black-and-white. Yuasa stated that the first Gamera film had a budget of about ¥40 million and that the film went "over budget a little bit". Due to the commercial success of the first Gamera the second film had an expanded budget that Yuasa stated was 80 million yen.
Gamera the Giant Monster was released in Japan on November 27, 1965. The film was a bigger hit than the studio expected, which led to a higher budget for the second film Gamera vs. Barugon.
An edited version of the film was released theatrically in the United States on December 15, 1966 as Gammera the Invincible. This version contained new scenes featuring American actor Brian Donlevy that were not in the original Japanese version. These included the scenes with the Alaskan army base, the Pentagon, and the UN headquarters. This version of the film was also shown frequently on American television in the 1970s.
The film was released on home video as Gamera in 1987 by Sandy Frank Film Syndication. This release contained the Japanese version of the film without the added footage from the 1966 U.S. theatrical release, and was entirely re-dubbed in English with a different musical soundtrack. However, the 1966 theatrical version featuring Brian Donlevy has also been made available on home video as well.
Vintage Home Entertainment released Gamera: The Ultimate Collection on May 17, 2005 which included a compilation of the entire film series. The film was last released by Willette Acquisition Corp. on Feb 17, 2015.
From contemporary reviews, Variety stated that Plan Z in the film was an "appropriate idea for Gamera, a film which can also be rated as Grade Z".
From retrospective reviews, AllMovie gave the film a positive review complimenting the films direction, special effects, and cinematography stating, "All in all, Gammera the Invincible is a solidly-crafted, engaging monster mash - just make sure you see the original Japanese version". It currently has a 20% "Rotten" score on Rotten Tomatoes.