This is the fifth volume in order of publication, and continues the story from Foundation's Edge. Trevize made his decision, but is uneasy because he does not know why he made the decision he made. He decided he needs to go looking for more. And his traveling companion, Janov Pelorat, is an historian who is interested in the mythology about a "planet of origin" called Earth, and wants to look for it. So they go off in search of it. As they look at old legends, they start finding very old planets, and it turns out Asimov has merged the Foundation universe with his Robot universe. The planets they find, at least at first, are a couple of the old spacer worlds: Aurora, and Solaria. Eventually they find Earth, but is radioactive. This was first introduced in Pebble in the Sky, then explained in Robots and Empire. Then they notice the Moon, which is unusually large, and investigate, where they find R. Daneel Olivaw, who has been manipulating everything for the last 30 millennia. After this Asimov could not think of anywhere to go with this story, so he did a couple of "prequel" books.

I now have this as part of a 7-book set of all of Asimov's Foundation novels in e-book form.