This episode shows further progress in the cracking of the German codes, including the extensive use of 'cribs' and the algorithm solving Bombe machine devised by Alan Turing. A reunion of WRENS recalls their memories of working the Bombes, locating the rotor wheels onto the machines (and cleaning the wire contacts when they stuck), and plugging up the stecker-boards from the crib menus. They used the following phonetic alphabet : Apple, Butter, Charlie, Dan, Edward, Freddie, George, Henry, Ingrid, James, Kenneth, London, Monkey, Nuts, Oliver, Peter, Quagga, Robert, Sugar, Tommy, Unicorn, Vinegar, Willie, X-ray, Yellow, Zebra. The information obtained from all these sources was of course 'ultra top secret' and had to remain so otherwise the Germans might reconfigure the Enigma. Winston Churchill referred to this intelligence as his 'Ultra.'