Aeons ago, a super-scientific culture known as 'Angels' had left incomprehensible relics all over the galaxy. Among these phenomena were the Stations, whereby human spacecraft could jump instantly from one part of the galaxy to another. And from them the brilliant Angel technology could be explored and exploited. One of these stations orbits the planet Kaspar, where the only other known sentient species outside Earth has been meticulously allowed to continue evolving in its own world of primitive ignorance. But suddenly Kaspar's mysterious 'Citadel' has become the vital key to repelling the fast-approaching threat. At what cost, though, to its native inhabitants ... and to the human residents of the orbiting Angel station?