By air Nelson and Crane pursue a mysterious submarine stealing nuclear missiles. Their craft is shot down and Crane is captured. Nelson, recovered by a Navy destroyer, is bent on destroying the sub, not realizing Crane is on board.

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.