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As the Great War began, the Royal Navy rushed to Orkney's great natural harbour, Scapa Flow. David Hayman uncovers the compelling characters of the little-known naval war - the cautious Admiral Jellicoe and the playboy Admiral Beatty. The story of great technologies and epic battles for control of the North Sea.

In the second of this two-part series, David Hayman explores Germany's World War One U-boat threat and the fascinating and dastardly ploys Britain used against the submarines.