In and of itself, this novel is relatively unimportant. It is the fourth (and last) book in the "Robot" tetralogy, but in terms of character and plot, has little to recommend itself. On the other hand, in terms of setting the stage for the final developments in Asimov’s idea of the positronic robot, it is critical.

Functionally, the novel does two things: First of all, it ties up any loose plot points left over from Robots of Dawn. Many of the main characters (such as Gladia, R. Giskard and R. Daneel Olivia) are leftovers from the earlier book, and the main one left is a descendant of Elijah Baley and his (literary) heir. And, insofar as this aspect of the book is concerned, I cannot say that it is terribly memorable for me. It has something of the travelogue nature of some of the later robot/Foundation books, and it has a goodly helping of sex—but it is not memorable.

What is memorable about this story , as I say, is its establishment of two key points:

The first is the final and most official reason for why the Earth became radioactive. The radioactive Earth features as a plot element in the three "Empire" novels, Pebble in the Sky, The Stars, Like Dust--, and The Currents of Space, and in all three, it is attributed to nuclear war. (This was a common expectation in the early 1950’s, when these books were written; it was only later that Asimov realized that from a physics perspective, it was unreasonable.) With the additional perspective of nearly forty years, Asimov felt a need to provide a more rational explanation for this phenomenon, and does so here.

More important is the formalization of something which has been implicit from the time, at least, of the first Susan Calvin story, "Liar!", namely, the "Zeroth" Law of Robotics. This simultaneously provides a culmination of potentials for the Laws of Robotics and sets the stage for genuine robotic influence in the Foundation books, and as such, as a brilliant stroke. Its great misfortune is to be trapped within a novel which is otherwise not one of Asimov’s best.