An American oil tanker, the SS Northern Star, is sunk in the North Atlantic by a German U-boat. The ship is commanded by Captain Steve Jarvis, portrayed by Raymond Massey, and his First Officer Humphrey Bogart as Joe Rossi, as they and several crew members abandon ship in a lifeboat that is rammed and sunk by the pursuing U-boat. After eleven days adrift on a wooden life raft, the survivors are finally rescued, gathering a hard-earned sense of relief and wary hope from the ordeal.
Back on shore, Steve spends a quiet moment with his wife, Sarah Jarvis, a relationship brought to life by Ruth Gordon. Meanwhile, Joe Rossi meets and marries singer Pearl O'Neill, with Julie Bishop delivering her performance as Pearl. The Northern Star survivors await new orders at the maritime union hall, and these lead them to a brand-new Liberty ship—the SS Seawitch—again under the command of Captain Jarvis, with Rossi continuing as First Officer.
The Seawitch is a comparatively modern, well-armed freighter, equipped with anti-aircraft guns and a dual-purpose cannon, crewed by trained Navy gunnery personnel. It sets out on a convoy bound for the Soviet port of Murmansk, carrying crucial supplies. The convoy is scattered by a ruthless U-boat wolfpack, forcing the Seawitch to improvise. The ship hides at the edge of the Arctic icepack to shake off pursuit, slipping into silence in a bid to outwait its hunter. Yet the chase resumes when the submarine locates the freighter again. During a tense encounter, Captain Jarvis is wounded, and eight crew members lose their lives, leaving First Officer Rossi to take the helm.
The relentless U-boat returns, determined to finish off the Seawitch. Rossi devises a bold ruse: he orders smoke, lights fires on deck, and issues an abandon-ship signal to draw the submarine near for a final strike. The German captain surfaces to finish off the crippled freighter with his deck gun, but the Seawitch turns the tables and rams the U-boat, sinking it in a dramatic and decisive moment.
With the threat vanquished, the Seawitch presses on to Murmansk, where it is greeted by a squadron of Soviet fighters. The crew is met with an enthusiastic reception on the docks, including a surprising sight: a crowd of female longshoremen who cheer the American vessel’s arrival. The warm welcome reinforces the sense of shared purpose and camaraderie among Allied seamen.
The film closes with a poignant nod to history: a speech excerpt from President Franklin D. Roosevelt—voiced by Jack Young—honoring the valor and essential contribution of America’s Merchant Mariners to the war effort. The famous line resonates across the screen:
> From the freedom-loving peoples of the United Nations to our merchant seaman on all the oceans goes our everlasting gratitude.
