Mysterious aliens make contact with earth just as humans are about to launch their first ships into space. The so-called Overlords put an end to all war and nationalism, hunger and poverty. At the same time, human ingenuity seems to stagnate as well, and the purpose of these Overlords, though seemingly benign, remains mysterious. 
 I won't give away the ending of this classic science fiction novel by one of the masters of the genre, but I will say a few words about it. First and foremost, this is science fiction of ideas. The characters and their relationships aren't particularly well fleshed out--in fact, some characters become our focal characters for a few chapters and then are never heard from again. It's really about the progress of humanity as a whole under the Overlords and, finally, an interesting take on our evolution and destiny. The ending is unexpected, thought-provoking... and ultimately a bit unsatisfying, at least for me.