John Mills stars as a British submarine commander in this Second World War drama. The crew of the Sea Tiger have their leave cut short when they are ordered to pursue and sink a new German battleship known as the Brandenburg. The crew's captain (Mills) is tasked with destroying the ship before it enters the Kiel Canal but he soon discovers that the battleship is heavily defended and is further away than first thought. Along the way the Sea Tiger enters German controlled seas knowing that once they succeed in their mission they may not have enough fuel to return home.