When the King of a small but friendly nation is assassinated, Seaview is assigned to take the young Crown Prince back home to assume his throne. Neither Nelson (who gets turned out of his own cabin) nor Crane are used to handling children. Help arrives in the form of a mysterious flute-playing old man who befriends the boy after he is picked up at sea.

Cybernetic genius Tabor Ulrich creates a perfect mechanical duplicte of Admiral Nelson. His plot: to make the crew of the Seaview they are fighting World War Three. In doing so, the sub will launch nuclear missles at the China, Russia and the USA. After the holocaust, Ulrich plans to take over and usher in "the age of the cyborg".

Things go wrong when Nelson and Crane test out a new automated system for Seaview.