A team of geophysicists investigating seismic activity on the seafloor discover that the islands of Japan, after suffering from massive volcanic eruptions and earthquakes, will be pulled into the ocean, killing millions.

Exciting fights between the police and a crime syndicate over the secret of poison gas.

While covering a test of guided torpedoes, two reporters believe they see what appears to be a strange-looking swimming creature. They investigate the matter further and discover that there is a race of fish-men living under the sea. The fish-men capture the pair and keep them prisoner in their underwater city.

When a giant stone statue on Wester Island is disturbed, the legendary monster Jiger appears and heads for Japan. Gamera tries to stop this new rival, only to be injured when Jiger lays eggs inside of him. As two boys in a submarine go on a dangerous quest inside of Gamera's body to save him, Jiger threatens the Expo '70 world's fair in Osaka.