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Joe Rossi is chief executive officer on the Merchant Marine vessel captained by Steve Jarvis.  When their ship is torpedoed by the Germans while traveling in the North Atlantic, Steve vows retribution.  After Joe, Steve and the other survivors are rescued, they return home to wait for assignment to another ship.  Steve's wife Sarah is glad to have him back, although she knows that waiting and worrying are part of being married to a seaman.

Joe heads for his favorite bar, where he meets singer Pearl O'Neill, whom he marries shortly before he is recalled to sea.  The men learn that their new ship, the Sea Witch, is to be part of an international convoy bringing supplies to Murmansk in the Soviet Union.  As the convoy heads into the open sea, it is attacked by several German submarines.  United States naval destroyers engage the submarines in battle, but one singles out the Sea Witch .  Steve orders his men to lure the submarine away from the convoy.  The submarine follows the Sea Witch just out of range of its guns, waiting for its chance to attack.  Joe suggests that they cut the engines and maintain complete silence, hoping that the submarine's sound sensors will not be able to track them.  The ruse works, but the captain of the submarine is able to determine where the ship is headed and radios a request for airborne bombers.

The airplanes and the ship engage in a battle, during which Steve is seriously wounded and several other sailors are killed.  When the submarine torpedoes the ship, Joe, who has been named acting captain, orders the men to start a fire, hoping to lure the submarine to the surface, where the ship's guns can hit them.  After the submarine surfaces, Joe orders the ship to ram it, and the submarine is destroyed just before Russian airplanes appear overhead to welcome the Sea Witch and her crew.