Walkers on the Sky (Fontana science fiction)

Plot

Sometimes the sky held only clouds, but at other times it could get quite busy. It could be full of sailing ships or bands of mounted warriors or even single figures strolling carefully across the empty air. From the viewpoint of those below they were either apparitions or gods, but in any case to be ignored. From the viewpoint of the sky walkers, those below were neither phantoms nor gods, yet certainly always beneath their notice. Both viewpoints were wrong. Because the time had come when one of the sky walkers was going to do the incredible--fall through. And when that happened, all hell was going to break loose. And did!