In the Arctic, an unknown aircraft is shot down by an American jet fighter. The aircraft crashes and its cargo, a low-level atomic bomb, explodes. The resulting cataclysm awakens a prehistoric monster that has the appearance of a giant turtle with teeth and large tusks. Japanese scientists on an expedition led by Dr. Hidaka nearby are given a "devil stone" by an Inuit chieftain, who explains that the creature is called Gamera.
Gamera destroys the American jet with his fire breath and escapes into the sea. On the way out, he destroys the research vessel that had transported the expedition, with the only survivors being Dr. Hidaka, his daughter Katherine, and Alex, a reporter who was covering the expedition. The monster heads to Japan and surfaces in Sagami Bay where Kenny, a boy releasing his own pet turtle on his father's orders, sees him. Gamera destroys the city of Fujisawa and a lighthouse. However, he also rescues Kenny from falling from the lighthouse, and then retreats back into the sea.
Scientists and government officials hold a conference to discuss killing the monster. Conventional weapons prove useless, and indeed it appears that Gamera feeds off the heat generated by such attacks. So experimental freeze bombs are employed. The detonation causes him to fall on his back. Having thus been rendered helpless, it's merely a matter of waiting for him to die of starvation. However, Gamera then pulls in his head, limbs and tail into his shell, emits flames from his front leg and back leg cavities, and flies away by rising up into the air and spinning around like a flying saucer.
Kenny and his family decide to stay with an uncle in Tokyo because they have nowhere else to go. Kenny 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 see an opportunity to use its nuclear 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 arrives at Haneda Airport, destroying the control tower, and proceeds to wreak havoc in Tokyo (including destroying the Tokyo Tower). Kenny and his family evacuate again, but he gets separated in the confusion.
Z Plan is still not ready, so the scientists plan to keep Gamera at bay in the meantime by confining him to an oil refinery. Dr. Hidaka has surmised that Gamera is gaining energy by consuming fires at the refinery and they will therefore keep shipping petroleum there by train to keep Gamera occupied for 24 hours. Kenny finds his way to the refinery and sneaks on board a train bound for Gamera, chased by the refinery chief. They are both thrown from the train when it explodes, but are unharmed and Kenny is sent on his way.
Z Plan is finally completed and Kenny sneaks on board a 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. Alex 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 into the nose cone of a giant rocket and launched on a one-way trip to Mars. The worldwide announcement of success extols the triumph of science over ideology. Alex and Katherine go off together and Kenny decides he is not sad, because he is going to be a scientist one day so he can visit Gamera on Mars.
- Eiji Funakoshi
- Harumi Kiritachi
- Junichiro Yamashita
- Yoshiro Uchida
- Michiko Sugata
- Yoshiro Kitahara
- Jun Hamamura
- George Hirose
- A sequence with Gamera attacking two strippers in a nightclub was shot, but cut from the final finished version of the film.
- This is the only film in the Gamera series that was filmed in black and white.
- This is the only film in the series in which Gamera does not fight another monster.
- This was the final kaiju movie to be filmed in black and white.
- This is the only film in the series with Gamera as the antagonist.
- Several of the characters go by Western given names in the English dub. Originally, Kenny was Toshio, Katherine was Kyoko, Alex was Aoyagi, and Nora was Nobuyo.
- Was nominated in The Golden Turkey Awards series for Most Ridiculous Monster. It lost to Robot Monster.
- The film was re-released in the United States in 1966 with additional scenes featuring western actors. This version was titled Gammera the Invincible and it became a RiffTrax presentation in 2018.