During World War II, the Allies were having trouble against the German Navy especially their U-Boats because of their highly sophisticated cipher machines. When a German U-Boat is damaged and is waiting for assistance, the Americans decide that this is an opportunity for them to get the machine. So they have one of their subs made up to look like a German, and plan to pretend to be the ship sent to assist them and take control of the U-Boat and take the machine and destroy the sub so that the Germans will not know that they have the machine. And everything went according to plan; they got the machine and transferred the crew to their sub and were about to leave when out of nowhere they're hit by a torpedo from the German relief sub. The members of the crew who were not on board the sub, proceed to board the German U-Boat and try to make it back. Now the senior officer is the sub's XO who's eager to be in command but his commander had told him earlier that he felt he was not ready because he might not be ready to make tough decisions. And now he has to, if he and the survivors are to survive.

The caption before the end credits, detailing the fact that the Royal Navy captured the first Enigma machine, was only added after an outcry in Britain, where it was believed that Hollywood was trying to claim the credit for the Americans (whose forces captured no German Naval Enigma material until 1944).

Marine Maj. Coonan:
How far down does this ship go?


Lt. Commander Mike Dahlgren:
Oh, she'll go all the way to the bottom if we don't stop her.

Factual errors: The Nazi emblem placed on the S-33 is conspicuously missing in later scenes. As shown in the movie, the code book which cracked Enigma during the Battle of the Atlantic was the long weather code book. This was only captured once - by the British. Subsequent attempts by the US failed as the book would dissolve on contact with water. The film does not purport to be telling a true story; a notice at the end acknowledges the real-life ships whose crews captured Enigmas.

Written by Sigmund Romberg, Oscar Hammerstein II
(performed at the wedding reception while Dahlgreen is talking to Tyler)