In 2020, an international space team, exploring Sigma Draconis, 19 light years from earth, discovers the remains of a highly advanced society that has left behind as its most spectacular artifact the largest telescope imaginable, carved & polished from a natural moon crater. Successive space crews determine that the native culture evolved & disappeared mysteriously after a mere 3000 years of existence. It's now 2028. Another mission reaches the planet with just one goal--to discover why the civilization disappeared--& with just one hope--that this knowledge will prevent the same thing from happening on earth. Exhibiting that rare sense of sf mystery that distinguished Arthur C. Clarke's 2001, John Brunner weaves a haunting tale of how 30 people attack the nearly insuperable task of unriddling the mysteries of a long-buried culture. Was it a fatal virus, an internecine war, a religion of lunatic brutality or a deleterious mutation that destroyed an entire civilization? All remains hypothesis until Ian Macauley unravels the riddle. But does it provide a solution to human problems & will the answer reach earth in time?