Pun Salad

The book is set a couple centuries after the events of The Robots of Dawn. The effort Elijah Baley set in motion in that book is in progress: after centuries of isolation, Earth is sending out "Settlers" to expand into the Galaxy. This is met with some resistance from the culprits in the previous book. They want to shut down Earth's expansion and leave the Galaxy for the "Spacers", the original colonists.

Worse, Elijah, being a Terran, has passed away. (He shows up in a flashback, though.) But the robots are still around: R. Daneel Olivaw and his de facto new partner, Giskard. Their task is to protect their mistress, Gladia, as she embarks on a mission to find out why all the humans have vanished from her old home world of Solaria. What they discover turns out to (of course) have galactic repercussions. Daneel and Giskard find themselves on dangerous ground, not only from human enemies, but also from the limitations of Asimov's Three Laws of Robotics.

Oh yeah: the book was written in 1985, and the closing scene is set up in such a way that it vitally depends on the hysterical reaction to (without spoilers) a notable event of the late 70s. Asimov clearly throught that event would have much more importance in the near future than it actually turned out to have. This doesn't ruin the book, fortunately, but Asimov was a far better storyteller than prognosticator.