Set six months after the events of ''Gamera: The Giant Monster'', a meteorite collides with the Z Plan rocket transporting the creature to Mars (planet). Now free, Gamera returns to Earth and destroys Kurobe Dam in Japan. At the same time three mercenaries are sent by Kano, a World War II veteran, to an island in the South Pacific to retrieve a huge opal he once found and hid in a cave. Despite warnings from the local villagers, the trio find and locate the opal, but one dies from a fatal scorpion sting. The second man, Keisuke, Kano's younger brother, is betrayed by his fellow expeditioner Onodera and apparently killed in a cave-in.