The cargo ship commanded by Captain Steve Jarvis (Raymond Massey) is sunk in the north Atlantic Ocean by a German U-boat. He and his first officer and friend, Lieutenant Joe Rossi (Humphrey Bogart), make it to a lifeboat along with other crewmen. When the Germans start filming their plight, they respond with rude gestures and are rammed. The men swim to a raft and are rescued after 11 days adrift. During their brief leave, Steve spends time with his wife Sarah (Ruth Gordon), while Joe meets and marries singer Pearl O'Neill (Julie Bishop). Then, it is back to sea on a new Liberty ship, on a convoy carrying vital supplies to the Russians at Murmansk.
The convoy is attacked by a wolf pack. There are losses on both sides, but the convoy commander is forced to order his ships to disperse. One persistent U-boat chases after Steve's ship, but loses contact in the night. Two Heinkel He 59 flying boats find the freighter and attack. Both are shot down, but the second crashes into the bow. Steve is shot in the leg during the battle; Joe has to take command. The U-boat sights the ship again and hits her with a torpedo. Joe orders the men to set fires and make smoke so that it appears as if the ship is sinking. When the submarine surfaces to finish her off, Joe rams and sinks it. The freighter then limps into Murmansk to a warm Russian welcome.