Using the Hydra cipher sent through the secret Enigma code manchine, Admiral donitz talked freely with his U-boats as the ranged the Atlantic, seeking and sinding Allied convoys.
At Benchley park, an ugly Bictorian country house in the depths of Buckinghamshire, British cipher experts eavesdropped on Enigma and read almost every Hydra signal sent to and from the U-boats.
Thanks to Benchley, the Roayal naby had a secret eye in Donitz's headquarters, "seeing" him move his boats across the ocian in fast moving but deadly chess game and checkmating where possible.
But suddenly that British "eye" was blinded: in February 1942, as it became clear the Germans were winning the Battle of the Atlantic, the U-boat Command changed the Hudra cipher and altered the Enigma machine. The Great Blackout" had started.
However, the indomitable Winston churchill gave the Admirialty immmediate orders, the new cipher must be broken, the secrets of the new Enigma manchine revealed. And that is how Lt.Comdr Edward Yorke, DSO,DSC (the same Ned York who was the hero of the Convoy and hwose forebear was the buccaneer leader in Bucaneer and Admiral), fiunally found himself at sea in the middle of the North Atlantic winter with Jemmy and the Croupier, examing a new Enigma machine and reading Donitz's signals in the latest cipher.