During World War II, successive tugboat British Captains sent on dangerous salvage missions pass-on the key to a home-port apartment, where a lonely Swiss-Italian young war-widow lives.
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.—Huggo
In wartime England, circa 1941, poorly-armed tugs are sent into "U-Boat Alley" to rescue damaged Allied ships. An American named David Ross arrives to captain one of these tugs. He's given a key by a fellow tugboat man, a key to an apartment and its pretty female resident. Should something happen to the friend, Ross can use the key.—dinky-4 of Minneapolis
American David Ross (William Holden), a former tugboat captain now in the Canadian Army, is hastily commissioned in the Royal Navy and assigned to rotating command of W88, a double-screwed rescue tug then in dry dock due to battle damage. His predecessor committed suicide. The slow, poorly armed tugs bring in "lame ducks," freighters crippled near Britain by German attacks. The main danger is from U-boats and aircraft.
David is reunited with an old friend, Captain Chris Ford (Trevor Howard), who commands another tug about to go on a mission. Chris takes David with him and they are attacked twice. That night Chris brings him home to his flat to meet his lover, Stella (Sophia Loren), who wears a wedding ring. She had been engaged to Philip Westerby, another tugboat captain, but he was killed the day before their wedding. A friend of Chris', Van Barger, took possession of the hard-to-find flat and Stella stayed with him. Knowing the extreme danger of his job, Van Barger then gave a copy of his key to the flat to Chris, so that Stella would be taken care of no matter what. When Chris chooses David to be the next in line, he tries to refuse, but his friend is insistent.
David's tug comes out of dry dock and he goes aboard to take his turn of command from the tug's other captain, Van Dam (Oscar Homolka), who gives him tips for survival in combat but warns him that his real enemy is the one within himself, fear. Shaken by his recent close call, Chris proposes marriage to Stella, who accepts. However, she has a premonition that he will not be coming back from his next mission. She is proved correct.
At first, David refuses to move in. When he eventually does, he is surprised that Stella does not share his bed, but as time goes by, she falls in love with him, unlike the others. She puts away her photograph of Philip, gets rid of the uniforms of David's predecessors, and takes off the wedding ring. She also leaves her flat for the first time since Philip was killed. Finally, she asks David to marry her, and he gladly accepts.
With the U.S. entry into the war, an American freighter becomes David's next assignment, even though it is Van Dam's turn. Its inexperienced crew sends out a continuous S.O.S., contrary to sealed orders, revealing the ship's position to the enemy. When David finds out the situation, he tries unsuccessfully to refuse what amounts to a suicide mission. Knowing his chances, he gives his key to the new captain of another tugboat, Chris's former mate, Kane (Kieron Moore).
David's tug is attacked by a U-boat and hit numerous times. He orders the crew to abandon ship, then rams the submarine. After being rescued, David hurries back to the flat, but Kane is already there, having told Stella that David was killed. When she sees him alive, she screams at him to get out, hurt to the core by his betrayal in passing on the key. Later, Kane finds David drinking his sorrows away, and informs him that Stella is leaving for London on the train. David does not arrive at the station in time to board the train, but vows to Kane that somehow after the war he will find her.
