Riddle of the Sentinels

Terran explorers have finally reached other stars. They have discovered no signs of other life. It appears that humans are alone in the galaxy. A robot scout ship unexpectedly finds ruins in a distant star system. The recovery of an alien space buoy confirms the existence of other life. A Terran cruiser is dispatched to investigate these startling developments. The Argonaut and her crew quickly learn that they are on a dangerous quest to solve the riddle of the sentinels.