On the night of 13/14 October 1939, the German commander of U-boat U-47, Günther Prien, steered past the sunken block ships and chains which inadequately protected the British naval base at Scapa Flow in the Orkney Islands. The U-Boat sank the old British World War I battleship HMS Royal Oak and then escaped into the North Sea. The loss of the Royal Oak was insignificant in naval terms though over 800 men perished with her, however this was a bitter blow to British moral. There have been two 'Phantoms' of Scapa Flow. The first was U-47 and how she slipped through the heart of the heavily defended base. The second was the insistence of Prien that he hit two warships that night. The British Admiralty admitted only to the loss of Royal Oak. The second phantom is that unknown ship claimed by Prien.This book investigates both phantoms beginning with a dramatic reconstruction of the German operation. The second part is the result of research in both Britain and Germany including much evidence gathered from surviving eye witnesses on both sides. In a series of appendices the various conflicting facts and opinions are laid before the reader, producing a remarkable picture of the actual events.