During World War II, tugboats conduct what are called salvage missions, picking up disabled ships. Not well equipped with weaponry, the tugs are sitting ducks for enemy fire. As such, the crew working the tugs have precarious lives, many with deep seated emotional problems. Before the Americans join the war, ex-American military man David Ross is assigned to captain a tug for the British military. He is shown the ropes by an old friend, Captain Chris Ford. Chris currently shares a flat with a young beautiful Italian-Swiss woman named Stella, who came with the flat and who lives a reclusive life there. Chris is the latest in a long line of tugboat Captains who have lived there, each who has found another person to take over the flat and the associated looking after of Stella if anything is to happen to him. That person is given a key to the flat, the key only to be used if needed. The first in the series was Phillip Westerby, to whom Stella was to be married before Phillip was killed. Chris, who, in turn, now loves Stella and wants to marry her, asks David to be the next in line. David reluctantly agrees. As David learns the pressures associated with his work, he begins also to understand the emotional turmoil that Stella has gone through, which changes his opinion about "the key". Stella's view of the key also changes with David.