In the dark winter of 1941/1942, Lieut. Commander Ned Yorke is ordered, by Churchill no less, to obtain a new Enigma machine and its Hydra cipher from the Germans. The British have the older, Mark II, Enigma, but only a Mark III will enable them to monitor German U-boat wireless messages, a crucial element in the Battle of the Atlantic.
Yorke assembles a couple of chums and 20 hands for a daring escapade: as apparent shipwreck survivors they're picked up in the North Atlantic by a U-boat, then overpower its crew and take it over, along with its new Enigma. Their initial success is marred, however, by lack of radio transmission and the real danger, and many adventures, are encountered on their voyage back to Britain.