The S. S. Claridon is scheduled for her five last voyages after thirty-eight years of service. After an explosion in the boiler room, Captain Robert Adams is reluctant to evacuate the steamship. While the crew fights to hold a bulkhead between the flooded boiler room and the engine room and avoid the sinking of the vessel, the passenger Cliff Henderson struggles against time trying to save his beloved wife Laurie Henderson, who is trapped under a steel beam in her cabin, with the support of the crew member Hank Lawson.

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.

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.

A killer-for-hire gets mixed up with a cast of wacky characters involved in a gold smuggling route from Beirut to Tokyo.