In 1940, the testimony of Chief Gunner's Mate Mike Mallory [Pat O'Brien] at a United States Navy Board of Inquiry about a fatal gun turret accident helps end the career of Lieutenant Tom Sands. The case is complicated by a forbidden romance: Sands and Mallory's sister, Myra [Jane Wyatt], are in love. After the inquiry, Sands resigns his commission and breaks up with Myra, telling her there is no future for them.
When the United States enters World War II, Sands, now an enlisted man, rejoins the Navy and is unexpectedly assigned to Mallory [George Murphy], to their mutual displeasure. They are ordered to man the guns of the freighter Sybil Gray, a duty that dampens any sense of glory about their service. Myra, meanwhile, travels with the convoy as a Navy nurse and comes ashore briefly to say goodbye to her brother—reuniting with Sands in a moment charged with history.
On board, Coxswain G. Berringer [Max Baer] immediately marks Sands as a troublemaker, turning him into a pariah among the ship’s crew. The voyage toward England unfolds with the constant threat of war as a German U-boat stalks the convoy. Escort ships finally drive the submarine away, and Doctor Lieutenant Commander Murray and Myra are brought aboard to perform surgery on James Bayless [John McGuire], who is seriously injured in the fighting. They stay on the ship to avoid delaying the convoy any further. A near encounter with a German pocket battleship in the fog underscores the peril, and Sands confesses to Myra that he still loves her.
Two German airplanes strafe and bomb the Sybil Gray. Myra is knocked out by falling debris, and Sands abandons his gun to carry her to safety. In the confusion, Berringer is fatally wounded, and the attackers are shot down. Yet the crew is left with a painful impression when they suspect that Sands might have been a coward.
A radio intercept intercepted by Babe Duttson's team reveals a German message, and Richard “Dutch” Kroner [Carl Esmond] interprets it, informing Mallory that a German U-boat supply ship is nearby. Mallory persuades the freighter's captain to change course and capture the vessel. Unbeknownst to the Americans, the German captain rigs a torpedo to explode after a delay, but Sands senses the danger and foils that scheme.
Then Sands disobeys Mallory's order to guide the German ship to Belfast, deciding instead that they can load unsuspecting U-boats with booby-trapped torpedoes. With Sands the only qualified navigator available, Mallory has no real choice but to agree. The plan runs smoothly for the first three submarines, but on the fourth, an officer recognizes Dutch as a famous anti-Nazi violinist, and the tension escalates into a direct confrontation. A second U-boat surfaces and joins the fight, and the Americans sink both submarines, though the hold of the supply ship is set ablaze. When Mallory goes to deal with the fire, he is overcome by fumes and is rescued by Sands.
After the action, Sands questions Mallory about his conduct during the battle, and Mallory concedes that the situation mirrors the events of the earlier inquiry—except this time there are survivors who could testify. Returning to the United States, the Board of Inquiry is reconvened, and Sands is reinstated as an officer.
