Using the secret motive power of a lost a lost flying saucer, physicist Micael Arnott, three companions and an escaped convict are flung into the void at eight times the speed of light to eventually land, after the oblivion of acceleration, upon a world that is both extraordinary and terrifying. Their machine disappears and they themselves also vanish one by one, Michael Arnott going first when he is on the verge of explaining the mystery of this far-flung world. That the planet is inhabited seems obvious from queerly designed spaceships glimpsed at intervals, all of them blazoned with a "Z", which is not so much an alphabet letter as a symbol of a master-race of scientists. In their efforts to solve the riddle of the world and system to which they have been hurled, the perplexed travellers gradually realise they are not only involved in an odyssey of space, but in a problem of Time as well. They are forced to the conclusion that, just as the first supersonic airmen paid a penalty of mental blackout for breaking the barrier of sound, so there is also a penalty for exceeding Fitzgerald's Law - namely that 186,000 miles per second is the ultimate possible speed.