The fifth episode in this incredible series explores one of the greatest contests of speed - the battle to break the sound barrier. Many people - scientists included - thought it could never be done. The test pilot's job was extremely dangerous and all too often deadly, in no small part due to compressibility, a scientific effect causing greatly reduced manoeuvrability when approaching the sound barrier. To combat this problem, the weird and wonderful aircraft that were designed to fly beyond the speed of sound boasted a great number of innovations, from ever-more powerful jet engines to moving tail-wings and razor-sharp wings. The race began during World War II when the British uncovered secret German plans for a 1,000mph aircraft and the Germans launched an extremely fast first-generation jet aircraft - the Messerschmitt 262. The British answer was to develop the M52, a revolutionary jet-powered, bullet-shaped plane designed to fly well in excess of the speed of sound, and the DH108 Swallow, a unique design sporting innovative swing-wings. The Americans entered the race towards the end of the war with the stunning Bell X1, which was powered by a combination of liquid oxygen and highly volatile alcohol: 'Like flying a streamlined bomb.' The race was characterised by controversy and extremely dangerous test flights. Speed Machines features stunning archive footage and talks to the men who designed and flew these incredible aircraft, including the first man to cause a sonic boom.