WARNING: Contains nothing but spoilers
The movie opens with Gamera arriving at Mt. Fuji during an eruption and then entering the volcano, as his previous films established that he has a love for heat and flames. The rest of the movie deals with a construction company trying to build a road going through a village in Mt. Fuji. Negotiations aren't making any progress, and things soon go downhill after a strange blue glow is seen emanating from a cave. A plane full of scientists is sent to investigate this phenomenon, but the plane is soon destroyed by a strange yellow beam that slices the plane in half.
When a young boy, named Eiichi, accompanies a reporter to the cave, what seems like a cave-in causes the reporter to ditch the boy. The reporter is soon eaten by a monstrous beast, Gyaos. After making his appearance, Gyaos tries to eat a fleeing Eiichi, but before he can teach the boy to wear some actual pants, Gamera appears and engages Gyaos in battle. Gamera saves Eiichi and drives away Gyaos by using his fire, although he almost loses his right arm in the process. After saving Eiichi, Gamera puts the boy on his back [!] and he slowly flies over to where all of the humans are. After Eiichi is safely rescued and in the arms of his family, Eiichi officially names the winged menace Gyaos.
Onlookers noted that Gyaos was driven away by Gamera's fire, and that Gyaos has only attacked at night. Numerous attempts are then made to sdestroy Gyaos, but to no avail. Finally, the moment that everyone has dreaded has arrived when Gyaos leaves Mt. Fuji and attacks Nagoya, devouring dozesn of people laying much of the city to waste. Gyaos is soon interrupted by Gamera, who almost manages to drag Gyaos underwater like he did Barugon, but Gyaos cuts his own leg off with his sonic beam in order to escape. Near the end of the second battle, Gyaos's head glows red as the sun rises, and the military takes note of this. When Gyaos's severed foot is found and brought back to a lab, it is discovered that it shrinks when exposed to sunlight and ultraviolet light. Gyaos's weakness has been confirmed.
More attampts are made to destroy Gyaos, again to no avail. these attampts include setting the forest on fire, which Gyaos soon douses with a yellow powder from his arm pits (...yeah, he does.), and a giant turn-table with a pool of artificial blood, which the military hopes will disorient Gyaos enough so that he won't notice the sun rising. Again, this fails. Meanwhile, back with the villagers, attempts at negotiating have finally broken down after the construction company decides that it is not worth building a road near the lair of a giant man-eating monster. The villagers, who were hoping to become rich by making the construction company increase their offer for the land, become dejected, and with Gyaos's determination to not die, all hope seems lost until Gamera reappears, once again engaging Gyaos in a duel to the death on Mt. Fuji. Gyaos is finally killed when Gamera drags him into the volcano, tusk to throat style. Similar Gamera's previous foe, Gyaos emits one last sonic beam before finally dying. Gamera soon rises out of the volcano and flies away, and the movie ends.
