The assassination of Doctor Sarton in Spacetown throws the community into disarray. Who would have an interest in eliminating the very person who campaigns for rapprochement between Earthlings and Spacers? Medievalists, who do not view the proliferation of robots very favorably? The Spacers themselves, ready to do anything to maintain their privileges? Detective Baley's problem, however, is not only finding a murderer, but above all getting there before his robot colleague R Daneel. Because this one is one of those androids with an ultra-perfected electronic brain, certainly created by man, but who are perhaps only waiting for the opportunity to take his place...