In 1940, David Ross, a sergeant in the Canadian Army, reports for duty at a British wartime naval port. There Wadlow, the head of operations, explains that David is to command one of the vessels in a fleet of tugboats that is assigned to rescue crippled British war ships. David is pleasantly surprised to be reunited with his old friend Chris Ford, who is also serving as a tugboat commander. While David’s boat is being refitted in dry dock, Chris invites him on a mission to rescue a “lame duck” ship. Unused to sea duty, David is sickened by the rough waters and the realization that the tug’s only protection is an antiquated machine gun. After a close call with a German submarine, the tug rescues the crippled ship and tows it back to port. With their mission completed, Chris invites David to accompany him to the flat he shares with a woman named Stella. Stella, who seems paralyzed by grief, responds to David’s complaint that his uniform jacket is too tight by going to her closet and pulling out another jacket with the name “Van Barger” sewn into it. As Stella hands the jacket to David, she stares at a photograph of her dead fiancé. Later, when Chris, unnerved by the perilous mission, begins to shake uncontrollably, he tells Stella that he is going drinking with David. At a bar, David, who is puzzled by the wedding ring that Stella wears, learns from Chris that her fiancé, tugboat captain Philip Wylie, was killed the day before their wedding. Philip had given the key to the flat to a tugboat captain named Van Barger, who took over the flat after Philip’s death. Stella, in despair, stayed on, and was passed to Chris, along with the key, after Van Barger’s death. When Chris gives David the key to the flat, David refuses to take it until Chris explains that he promised Van Barger that he would pass it along to another tug man who would look after Stella. Upon returning to the flat, Chris tells Stella that he is pessimistic about surviving the war and asks her to marry him. Stella, who identifies all tug men with her dead fiancé, accepts. That night, David checks into a hotel where he occupies a bed vacated by a recently deceased tug man. The next morning, David meets Van Dam, the captain with whom he will rotate command of the tug. The seasoned Van Dam advises David that “the enemy within is the one that matters.” After meeting his crew, David takes the tug out for maneuvers and gradually earns their respect. At the flat that night, Stella tells David that she and Chris are to be married. To celebrate, Chris opens a bottle of wine but accidentally shoves the cork into the bottle, thus splattering red wine all over his shirt. Stella reacts with horror because the wine reminds her of blood. The party comes to an abrupt end when Chris is called to duty and Stella, who has not left the flat since the death of her fiancé, insists on accompanying him to the pier, where she breaks into tears. David is also dispatched to rescue a burning freighter, and when he returns to port, he learns that Chris has been killed. With Chris’s demise, his first mate, Kane, is promoted to captain. When David tells Stella of Chris’s death, she impassively asks to stay on at the flat until the end of the month. After David insists that she keep the flat, she fatalistically responds “they are all the same,” leading David to believe that Stella is a common prostitute unable to love anyone. Some time later, when the gun on David’s tugboat once again jams, David demands that it be replaced, prompting Wadlow to tell him that they will never get new guns and that the German submarine that has been menacing his tug is a training ship using the tug for target practice. The news unsettles David, and that night, he returns to the flat drunk and announces “here comes another one.” After putting David to bed, Stella sleeps in the other room, and the next morning, David has the key duplicated. After returning from another rescue mission one night, David has a nightmare, and as Stella comforts him, they kiss. Soon after, Wadlow phones the flat with the news that the United States has entered the war. Elated by the news, David kisses Stella, who then calls out Philip’s name and breaks down. After David goes to report to headquarters, Stella slips off her wedding ring and leaves the flat. When David returns, he finds that the old uniforms have been removed from the closet. Just then, Stella comes home laden with packages. Begging her never to leave again, David hands her the extra key and proposes. On Christmas Eve, Stella gives David a note that says “I love you.” Later, when David uses a newspaper to light a fire, Stella becomes distraught when a photo of a tug boat on the front page goes up in flames. Their celebration is interrupted by air raid sirens, sending them scurrying to a shelter, and when they return to the flat, Stella is unsettled to see the charred remains of the newspaper. When David is called to duty, Stella begs him not to go and he defiantly asserts that he is “not like the others.” At headquarters, David listens in dismay as an inexperienced American freighter captain breaks the code of silence and broadcasts an SOS over the radio. Aware that the broadcast has alerted the enemy to the ship’s position, thus turning the assignment into a suicide mission, David refuses to go, but the silent disapproval of the staff forces him to reconsider. As he stands on the ship’s bridge, David, aware that he is facing death, sees Kane walking on the pier below and tosses him the key. At sea, a German submarine surfaces and shells the tug, setting it on fire. After ordering his crew to abandon ship, David stays aboard to ram the submarine and jumps overboard just before the collision. At headquarters, Kane hears a radio report that the tug has blown up and goes to tell Stella. When he enters the flat, Stella is devastated that David would have treated her like all the rest and turned the key over to Kane. After David and several of his crew are rescued by the freighter and returned to port, he rushes to the flat. Disillusioned by David’s breach of faith, Stella refuses to forgive him and orders him to leave. Soon after, Kane finds David in a bar and tells him that Stella has boarded a train to London. Running to the station, David bursts through the gates and jumps onto the train as it pulls away from the platform. When he walks into Stella’s compartment, she smiles.