An old freighter single-handedly takes on a Nazi war fleet.

At a Navy court of inquiry in Washington, D.C., Chief Gunnersmate Michael Mallory testifies that the negligence of Lieutenant Thomas L. Sands caused a lethal explosion aboard their ship. Protesting his innocence but unable to clear his name, Tom resigns his commission and renounces his sweetheart, nurse Myra Mallory, who happens to be Mike's sister. When war is declared after the bombing of Pearl Harbor, Myra enlists as a Navy nurse, while her brother is given command of a gunnery crew assigned to protect a Merchant Marine ship on its voyage across the Atlantic to Belfast. Tom, who has rejoined the Navy as an enlisted man, is assigned to Mike's crew. Other crew members include Joe "Babe" Dudson, an eager young seaman; Tarriba, a Cuban who feels a debt to the U.S. for liberating his country; Richard "Dutch" Kroner, an Austrian violin virtuoso who is wanted by the Germans; and Berringer, a former boxer. Once they are settled in their quarters, the men begin to gossip about Tom's dubious past. Soon after leaving port, the ship is attacked by a German submarine, and James Bayliss, one of the recruits, is wounded. Because the ship has no medical facilities, the captain signals for a medic from the hospital convoy, and Myra accompanies the doctor on board. As a thick fog spreads, the ship receives a report of a German battleship in nearby waters. With danger imminent, Myra insists upon seeing Tom. Mike delivers her message, but warns Tom that Myra will never be happy until she forgets him. Consequently, when Myra tells Tom that she still loves him, he makes up a story about being in love with someone else. The next day, as Dutch plays a waltz on his violin, German dive bombers attack the ship and Myra is knocked unconscious by some falling debris. Tom leaves his post to carry Myra to safety, but when Berringer, the only man who can attest to Tom's motivation, is killed in the attack, the others accuse him of cowardice and abandoning his post. Soon after the attack, Babe, an amateur shortwave radio operator, unscrambles a German radio message from a submarine on its way to rendezvous with a German supply ship. Mike convinces the captain to intercept the ship, and after the Americans fire a shot across the ship's bow, the Germans surrender. Mike and his crew then board the German ship, but Tom, suspicious of the ease with which the Germans surrendered, cautions Mike to hold the crew on board as insurance. This frightens one of the Germans into confessing that a torpedo has been set to explode. After disarming the torpedo, Mike asks Tom to navigate the ship to Belfast. As an alternative, Mike suggests arming the torpedoes and delivering them to the German subs to detonate. Donning German uniforms, the Americans sink several subs until their ruse is discovered. Attacked by two submarines at once, the American gunners sink one of the boats when the ship's magazine catches fire. After Tom risks his life to pull Mike from the flames, the Merchant Marine ship comes to the rescue and sinks the other submarine. Tom's courage forces Mike to realize that he has misjudged him, and Tom is reinstated as an officer when the ship reaches land. After he and Myra reconcile, Tom says goodbye at the Naval base, and then marches off with Mike on a new mission.