Capturing Enigma: How HMS Petard Seized the German Naval Codes

Capturing Enigma reconstructs the October 1942 boarding of the sinking German submarine U-559 in the eastern Mediterranean. It was this action that recovered the Enigma cipher material Bletchley Park needed to break the German navy's upgraded four-rotor U-boat code.

Harper follows the British destroyer HMS Petard through its wartime service. He focuses on the night two of its crew, Colin Grazier and Tommy Brown, swam to the abandoned U-559 and passed code books and cipher settings up through the conning tower before the submarine sank under them. The intelligence recovered ended a ten-month blackout in Allied U-boat tracking and is credited with turning the Battle of the Atlantic. Grazier and Brown's actions were kept secret at the time under the wartime restrictions on Ultra intelligence, and the two men were rewarded only with the George Cross rather than the Victoria Cross that had been proposed. The book also covers Petard's broader Mediterranean service beyond the single action named in its title.

Harper served as a coder aboard HMS Petard during the ship's second wartime commission and later worked for two decades as a Daily Express foreign correspondent. Capturing Enigma draws on his own service history and on interviews with surviving crew, and sits alongside his other wartime narrative histories, Miracle of Deliverance and Last Sunset.