The Seaview finds a sunken German U-boat while mapping the ocean floor. Sunk during WWI in the north Atlantic, it is now somehow in the south Pacific. Its captain boards the Seaview with Capt. Crane as his prey.

The Seaview is returning to the location of a sunken ship to recover its secret cargo. There are a number of other sunken ships in the area, and the thing responsible for all of them soon threatens the Seaview.