Ivan Zhilin, posing as a writer working on a novel, visits a seaside resort city to investigate a series of mysterious deaths. Zhilin's role as an undercover agent becomes apparent to the reader only gradually and is not brought into the open until the final chapters of the novel.

While being given a tour of the city, a tourism official tells Zhilin that he will get no work done, as he will be distracted by the "twelve circles of paradise" found in the city. These include the Fishers, which provide thrill seekers with situations of extreme and potentially fatal terror, the Shivers, which electronically induce pleasurable dreams to large crowds of people, and the Society of Patrons of Arts, who procure priceless works of art and ritualistically destroy them. The culture of this city has become utterly decadent, the product of an age of universal affluence. Zhilin refers to the present state of the world as "the age of the boob" where the highest priority is placed on orgiastic pleasure and staving off boredom, to the neglect of culture, education and scientific progress.
The ultimate expression of the decadence of Earth culture is the mysterious "slug", which is apparently responsible for the deaths that Zhilin is investigating. At first Zhilin believes it to be some sort of narcotic, distributed by gangsters with secret laboratories and trafficking networks. Zhilin progressively finds clues that lead him to Peck Xenai, his former classmate and the last surviving member of his international unit that fought the Fascists some years before. Peck, however, is physically ravaged by alcoholism and the use of "slug" and does not even recognize Zhilin when he finds him. Zhilin succeeds in getting a "slug" from Peck, in the form of a small silver electronic component. What Zhilin finds when he plugs the "slug" into his radio receiver and lies in the bathtub causes him to rethink the entire situation.

"Slug" turns out to be a way of generating an artificial reality significantly more intense than normal reality, to the point where there is virtually no comparison between our reality and that of the "slug". People become addicted to it and spend increasing amounts of time unconscious in their bathtubs until it kills them by nervous exhaustion or brain hemorrhages. This is "the final circle of paradise".