A British Sub goes missing at the end of the war leaving only one crew member surviving. Everyone believes it lost to the bottom of the Baltic Sea, then 40 years later it reappears without a crew. The British government send for the the surviving crew member (David Jason) now a marine biologist. With a new crew and Jason then proceed to trace the last know happenings on the sub. But this submarine has a life of its own and the new crew soon start showing characteristics of the old crew.