Andre Norton has no peer in his chosen field of science fiction for teen agers. This time his story involves an expedition in both time and space, as some young scientists- and a chance involved "rider for the Double A", Travis, who had been a student in the field of archaeology, set forth on a journey to repossess a lost space ship. Where their journey takes them provides breathless adventures in other worlds, with other-world creatures. Ingenuity, basic knowledge flexibly applied, and dauntless courage provide the handholds for success and a safe- though somewhat shattered-return. There's a hint of racial antagonism- a chip on the shoulder attitude- at the start, for Travis is an Apache but it has a negligible part in the story, though Travis' background and training stand him in good stead.