John Mills sets off to convince a sceptical Admiral (James Robertson Justice) of the merits of "midget" submarines in active warfare. His intention is to use these tiny craft to lay high explosives underneath the hull of the mighty Nazi battleship "Tirpitz" that is safely anchored in a steep-sided Norwegian fjord. John Gregson and Donald Sinden join him on the mission deep into the heart of enemy territory fighting both the elements and the novel technology. You get a sense of he claustrophobic conditions - and of the close teamwork that engenders, as well as the fragility of their craft…