Lieutenant Ward Stewart, Tyrone Power, commands a PT boat and earns a hard-won reputation after he sinks a U-boat and rescues a lifeboat full of survivors. The act of bravery underscores his skill, yet returning to port he is transferred to the submarine USS Corsair, a move that feels like a step down in rank and prestige, though the Navy is stretched thin for submarine command. So, for now, Stewart becomes an underwater warrior, adjusting to a different kind of warfare and the stricter discipline of a submerged crew.
At the submarine base in New London, Connecticut, he meets his new captain, Dewey Connors, Dana Andrews. The encounter is tense and uneasy; Stewart's enthusiasm for the undersea life clashes with Connors's practical, wary approach. The two men are aware that a clash may be coming, even as they prepare the Corsair for the dangerous assignments ahead.
Stewart secures a weekend pass before the Corsair sails on a mission. Connors uses a brief leave to visit his girlfriend, the New London school teacher Jean Hewlett Anne Baxter. Hewlett is pulled into a difficult obligation when she has to step in for another teacher and chaperone a group of honor students on a trip; Connors declines to accompany her, sensing the risks of wartime separation. On the same journey, Stewart and Hewlett cross paths, and the two meet aboard a train bound for Washington, D.C. A reluctant start gives way to a mutual attraction as Stewart begins to romance her, a relationship that will complicate the men’s professional rapport.
Dinner with Connors and Hewlett becomes a turning point. Hewlett, recognizing the seriousness of Stewart's feelings, suggests marriage as a solution to the strain, while Stewart explains that he is waiting for a promotion to commander that would allow him to provide for her. The romance deepens, even as Connors remains aware of the pull between duty and affection. The two officers’ personal tensions simmer, but the war ultimately requires them to put aside their resentments.
The DC briefing reveals a larger war enterprise: a secret island supply base for the Q-ship sunk earlier and other raiders. Connors is assigned the mission to locate and destroy this base, while the Corsair sails to disrupt as many German ships as possible. Jean Hewlett watches from the shore as the Corsair slips away, a quiet, heartbreaking moment that underscores the cost of war on personal life.
The hunt leads to the sighting of a German coastal tanker. Rather than sink it outright, Connors and the Corsair shadow the tanker through a harbor’s anti-submarine nets. A landing party of eight is organized and put ashore to destroy as many German munition stores as possible within 30 minutes, while the Corsair stays on the surface to sink other ships in the harbor. Among the landing party are Chief Mike 'Mac' McDonnell [James Gleason] and the Messman Oliver Jones [Ben Carter], who must improvise under pressure. The team’s raid is a remarkable success, but Chief McDonnell pays the ultimate price, sacrificing himself to cover the retreat of Stewart and Jones from a squad of German riflemen.
The battle ends with Connors taking a hit, yet he refuses to abandon the mission. He submerges the Corsair and heads out to sea, reinforcing the image of resilience under fire. In the aftermath, the two submariners reconcile their differences and renew their focus on the mission and on Jean. The Corsair returns to New London, and in a quiet, hopeful coda, Stewart marries Jean Hewlett.
