Set after the events of Gamera, a meteorite collides with the rocket transporting the creature to Mars. Now free, Gamera returns to Earth At the same time three mercenaries are sent by Kano, a WWII veteran, to an island in the South Pacific to retrieve a huge opal he once found and hid in a cave warnings from the local villagers, the trio find and locate the opal, The second man, Keisuke, Kano's younger brother, is betrayed by his fellow expeditioner Onodera back to Japan, Onodera accidentally leaves the opal exposed to an infrared light. incubates the opal and a monster Barugon eventually hatches. Growing to enormous size very quickly Japanese defense ministry, and suggest a plan takes on Gamera.