An attack by a British destroyer on a German U-boat in the Eastern Mediterranean in October 1942 altered the course of the entire war. The capture of secret German Enigma coding material from U-559, at the cost of the lives of two of HMS Petard's crew, enabled Bletchley Park's codebreakers to crack a new Enigma system introduced to cover a maximum U-boat offensive. After ten critical months with no U-boat signals intelligence available, this was the crucial factor in defeating Hitler's Atlantic U-boat wolf packs before they could starve Britain into defeat in the winter of 1942-3. Had it been possible to release the facts of her triumph at the time, the name of HMS Petard would surely have ranked alongside that of Nelson's Victory in the annals of British history. Because of the top secrecy about Ultra messages that lasted nearly four decades, the Petard's achievement remained unknown, her fame unrecognized. This text is the true story of how HMS Petard attacked and captured U-559 in the darkness of a Mediterranean night. With the use of vivid eyewitness accounts, the author describes how two of her crew swam across to the sinking, abandoned U-boat (ahead of a boarding party in a whaler) went below and passed key signals documents up the conning tower before being trapped when U-559, leaking from damage caused in the Petard's earlier depth charge attacks, sank. Both men were later recommended for posthumous awards of the Victoria Cross but the Admiralty, concerned this might draw unwanted attention from German Intelligence, instead ordered posthumous awards of the George Cross, the highest civilian award for bravery.