A series of volcanoes erupt in Japan. The eruption at Mt. Futago (in Shizuoka Prefecture) attracts Gamera, whose arrival is witnessed by a young boy named Eiichi. Gamera then climbs up and into the volcano. A research team is dispatched to the volcano to find Gamera and study the effects of the eruption. Meanwhile, Chuo Expressway Corporation is building a roadway nearby, but local villagers refuse to leave.
The research team's helicopter is destroyed by a sonic beam emitted from a cave in the mountains. Reporters are informed that no bodies were found, the culprit is not Gamera or the volcanic eruption, and an announcement will be made soon. One of the reporters, Okabe, leaves for the site, and he and the roadcrew foreman, Shiro Tsutsumi, arrive at a protest area simultaneously. Okabe sneaks through the barrier, and Tsutsumi and his crew are turned away, as are the villager protesters, by the arrival of a young woman.
The villagers return to inform the village headman of the happenings. The protests are a ploy to get more money for the land. The young woman is revealed to be Eiichi's older sister. The crew returns to find the work camp destroyed and a strange green glow coming from the mountain nearby. The work crew goes to investigate. Eiichi finds Okabe in the woods near Mt. Futago recording the same light the workmen saw. Okabe convinces Eiichi that it might be Gamera and the two make their way to a cave. A cave-in starts and Okabe runs away, leaving Eiichi in the cave. Upon exiting the cave, Okabe is eaten by a giant monster, which is later identified as Gyaos.
The workmen enter the cave from a different opening and Eiichi makes his own way out of the cave, where he discovers Gyaos and is trapped by a falling rock. Gyaos grabs Eiichi and Gamera appears. Shiro and his crew arrive just in time to see a battle between Gamera and Gyaos, during the course of which Gyaos drops Eiichi, who is rescued by Gamera. Gyaos injures Gamera with her supersonic beam, but is forced to retreat after several blasts from Gamera's fire breath. Gamera rescues Eiichi and brings him safely to a nearby amusement park. Shiro uses the Ferris wheel to retrieve Eiichi from Gamera's back and Gamera flies away.
Eiichi is interviewed about his experiences by Dr. Aoki and the Countermeasures Group, who have set up headquarters in the Hotel Hi-Land. It is Eiichi who calls the new monster "Gyaos" because of the sound ishe makes. Dr. Aoki explains Gyaos' abilities and that she was awakened by the volcanic eruptions. A squadron of aircraft attack Mt. Futago and Gyaos destroys them. Eiichi calls to Gamera, who is tending his wounds at the bottom of the sea. Gyaos attacks at night and all of the cattle in the village run away. During a meeting the next day, the villagers become divided on whether to sell their land or not because of Gyaos. Tsutsumi's entire crew, save two, also quits because of Gyaos. A rustling in the bushes scares the remaining men, who think it is Gyaos. Instead, Eiichi emerges and tells Tsutsumi that Gyaos only comes out at night. Tsutsumi reports this to Dr. Aoki and the defenders use light to make it too bright at night, without success. Gyaos annihilates the Japanese Self-Defense Force with a blast of wind and flies south to Nagoya.
Gyaos wreaks havoc in the city, and the people gather at the Chunichi Dragons stadium, where the lights have all been turned on. Gamera shows up and they battle in the skies over Nagoya. Gamera gains the upper hand because Gyaos' beams cannot penetrate Gamera's shell. Gyaos then extinguishes Gamera's flame jets with a yellow vapor. When Gamera hits the water, he bites Gyaos' foot and tries to hold her there until sunrise. Gyaos then severs her own toe and flies away. The toe is found washed up on the beach by some workers and is eventually brought to Dr. Aoki, where it is remarked that it has shrunk considerably since its discovery.
Further experiments reveal that ultraviolet light causes the severed toe to shrink. Therefore, if Gyaos is out in the sun too long, it will die. Meanwhile, Gyaos has retreated to its cave and regrown its toe. Eiichi, along with his sister, bursts into the planning meeting and inadvertently gives Dr. Aoki the idea he needs: they will lure Gyaos out at night, and immobilize her by making her dizzy using the rotating platform on top of the hotel.
The Defense Force constructs another platform on top of the existing one, with a giant bowl of artificial blood on it. With the help of Tsutsumi and his crew, they also build a viewing shelter. A delegation of villagers then appear and tell Tsutsumi they will no longer oppose the expressway. The headman appears and there seems to be some disagreement between the two sides. Gyaos is lured out, but the plan ultimately fails when the substation powering the motors explodes. Gyaos then destroys the hotel, extinguishes the substation fire with her vapor and flies away.
Because of Gyaos, the expressway is being rerouted, and the villagers, who were told by the headman to hold out, can now no longer sell their land. The villagers blame him and Eiichi comes out of the house, throws a tantrum, and berates the villagers for their greed. His sister also tells the villagers that the headman was acting in their best interests and the villagers leave. Back inside the house, Eiichi tells his sister that Gamera would finish Gyaos and that starting a forest fire on Mt. Futago will get Gamera to come. The headman goes to Tsutsumi and explains the plan. Tsutsumi tells the headman there will be a lot of money lost because of the destroyed trees, but the headman believe that Gyaos was sent as punishment for their greed.
Tsutsumi and his crew use their construction equipment to prepare the area for the fire and an airstrike starts it. Gyaos appears and puts out the fire. The second time the fire is started, it attracts Gamera. A fierce battle ensues, which ends when Gamera immobilizes Gyaos and flies her to Mt. Fuji, where he then drags Gyaos into the crater and flies away.Kojiro Hongo as Shiro Tsutsumi
Kichijiro Ueda as Tatsuemon Kanamura
Reiko Kasahara as Sumiko Kanamura
Naoyuki Abe as Eiichi Kanamura
Taro Marui as Mite-no-Tetsu
Yukitaro Hotaru as Hachikō
Yoshiro Kitahara as Dr. Aoki
Akira Natsuki as Self-Defense Force Commander
Kenji Oyama as Police Division Director
Fujio Murakami as Dr. Murakami
Koichi Ito as Road Corporation Director
Teppei Endo as Road Local Affairs Director
Shin Minatsu as Mitsushige Okabe
Teruo Aragaki as Gamera
Gamera vs. Gyaos was released theatrically in Japan on March 15, 1967. The film was never released theatrically in the United States, and was released directly to television by American International Television in 1967 as Return of the Giant Monsters. It was released to video and reissued to television as Gamera vs. Gaos in 1987.
The film appeared twice on Mystery Science Theater 3000. Its first appearance was in 1988, when MST3K was still a local program airing on KTMA. Its second appearance was in 1991, after it became a nationally-aired program.