A futuristic thriller set in the London of 2013. A hardboiled woman detective is assigned to catch a mass murderer who slays only those who are, or might easily become, mass murderers. Although she is sorely tempted to let him continue his obsession, she becomes interested in the killer; he quotes Ludwig Wittgenstein in his notes to her, argues philosophy over an untraceable phone line, and seems altogether an unordinary sort of criminal.

Philip Kerr's novel is not directly libertarian, but it asks questions that libertarians should ask themselves. For instance: if a gene is discovered that marks its carrier a possible mass murderer, what is society's ethical response to those so marked?