Macy stars as the COB (chief of the boat) on a fictitious US Navy submarine, the USS Swordfish (although there was an actual USS Swordfish that served the US Navy in World War II, it is not the same one depicted in this film). While on patrol in the North Atlantic, the ship is ambushed by two German U-boats. Swordfish destroys one of them, but a torpedo fired from the second one, the fictitious German submarine U-429, critically damages the American submarine. Eight members of the American crew escape and are held prisoner by U-429. However, the American captain had previously contracted meningitis and brought it on board. Within days, over half the German crew, as well as some of the Americans contract the disease and die. Although the Germans and Americans begin to work together to keep the boat operational, another U-boat and an American destroyer threatens to overwhelm their tentative truce.