From MORSE to ENIGMA, ULTRA to PURPLE, Room 40 to Bletchley, the story of secret signals in both of the World Wars. Communicating in the chaos of war is complicated, but vital. Signals intelligence makes it possible. In World War I, a vast network of signals rapidly expanded across the globe, spawning a new breed of spies and intelligence operatives to code, decode, and analyze thousands of messages. Signalers and cryptographers in the Admiralty's famous Room 40 paved the way for the code breakers of Bletchley Park during World War II. In the ensuing war years, the world battled against a web of signals intelligence that gave birth to ENIGMA and ULTRA, and saw agents from Britain, France, Germany, Russia, America, and Japan race to outwit each other through infinitely complex codes. For the first time, the secret history of global signals intelligence in the world wars is revealed.