"The Threshold of Space" looks back behind the cockpit of time to see new aircraft break more records in air-bound achievements. Speeds reach twice the speed of sound and beyond. There is a new phenomenon called "Inertia Coupling" which threatens to halt progress in the avenue of high speed flight by rendering the controls of the aircraft temporarily useless when flying at these high speeds. Spinning out of control became a common occurrence but soon this problem is rectified and once again velocity begins to reach new heights. The fastest plane featured, the Orbital X15, reaches Mach 6.7 or a speed of 4500 mph.